generic shopping guide

SHOPPING SMARTER IN SL
A lot of you guys make shitty avatars, and then have to go back and redo said shitty avatars after people inform you about it. This is a guide about how to shop for items that are decent quality to begin with, instead of wasting money buying things you won't use.
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1) LEARN SOME TERMINOLOGY AND HISTORY.
It's 2018, and SL is horrendously old. There's so much content that is piss poor by today's standards, but is still on the MP long after the person who made it left. Knowing a little bit about the history of SL features/building techniques will let you easily discern what items are the most up-to-date.
I. PRIMS: In the beginning, there were cubes, spheres, cones, and other simple shapes, and that was it. You can use some parameters to get fairly fun shapes out of these - called prim torture. However, for most uses, these are outdated.
Acceptable uses: Simple transparent objects, the "root" (parent, base, the thing that's yellow when you click on the whole object) prim for larger linksets, low-impact physics shapes, simple shadow planes under objects, simple spheres, skydomes, flexible objects, shit like that. Prims are great for building quick objects inworld, and unless brutally tortured, they retain their low land impact at any size.
Avoid: Complex buildings made solely out of prims. Furniture made out of prims. Armor or accessories made solely out of prims. God forbid, an avatar/head made out of prims. What are you doing. Hair composed of prims, or worse, hair composed of prims AND flexis.
II. SCULPTS: Often confused for prims by users too new to remember the era when sculpts were all we had. These are shapes defined by small images - called sculpt maps - that encode the positions of vertices in an object. Imagine having a simple square plane, or a cylinder, or sphere, and being able to push and pull it around like clay. There isn't a ton of detail that can be made with sculpts; many furry sculpt heads, for example, need one piece for the muzzle, and one for the rest of the skull. You can tell when objects are sculpted when they load in as spheres/elliptical shapes at first, and then as their proper shape.
Acceptable uses: Large objects for sim decor- sculpts retain their land impact at larger sizes. Particularly well-made sculpts (these will look practically identical to mesh, except when loading in, and can usually be found as old products of stores that make great mesh currently.) Various bits and bobs that you absolutely cannot find a mesh equivalent for. Parts that you absolutely intend on taking apart and using individual bits of.
Avoid: Sculpted items composed of a bajillion sculpted parts to make up for lack of detail. Old sculpted furry avatars. Items with mesh updates or equivalents. Sculpted long hair. Sculpted short hair with a billion pieces. Clothing. Honestly, if you don't know what you're doing yet, sculpts aren't the best option for you.
III. MESH (unrigged) : Mesh finally gave users the abiliity to upload just... plain old 3d models. Unrigged mesh can be moved, stretched, rotated, and attached to various body parts, just like prims and sculpts can be.
Acceptable uses : Most anything that you want to attach to just one part of your body (say, horns on the head, arm armor on the forearm, etc). Decor. Furniture. Buildings. Hair! The possibilities are kind of endless here.
IV. MESH (rigged, classic) : Wow! Finally we were able to make clothing and bodies that bend smoothly at joints rather than splitting them into parts and attaching them to each attachment point. At this point, things didn't work with all sliders on the avatar though (fat and muscle for instance), and rigged mesh can't be moved around.
Acceptable uses : Long hair. Various fantasy parts, like legs and shit. Clothing (though if you have a mesh body, you'll want to read on.) Various bodies. Animal avatars.
Avoid: Skintight/tight clothing, especially if you want to use it on a fitted mesh body.
V. MESH (fitted) : Mesh rigged to specific bones can react to more body sliders. This is where mesh bodies really took off. Please note that if you have a mesh body you'll usually need to find clothing for that body if you want the clothing to be fitted also - because you can't use sliders to adjust only one of them into fitting the other, you shouldn't shy away from coats or other non-skintight items that are not fitmesh.
Acceptable uses: Mesh bodies. Clothing for mesh bodies. Skintight clothing for mesh bodies. Clothing for default bodies, even. Most of the same things that regular rigged mesh is useful for. Doesn't seem to work well with animal avatars because of joint positions/deforms.
Avoid: You will know what you're doing when you want to actually avoid fitmesh.
VI. MESH (attachment point rigged) : Found mainly on not up-to-date animal avatars, the Solarian, ONA Kobold/Sergal, and centaur bodies, this is a very rare kind of rigging. It uses the bones meant as attachment points in order to animate some part of the body (such as a tail or ears). There is no need for anyone to really use this technique any longer, and it's quite hard to do in the first place. Usually, you'll need to undeform/relog after changing out of these avatars, as they can do a number on your skeleton.
Acceptable uses: Products without a better and newer bento alternative. Keep in mind that the technique isn't necessarily "better" than bento - it's both rigging to bones - but bento is far easier to work with and leads to better products from the same creator. IE: Solarian is still really good, as is the Kobold. BC centaur is still a lot easier to customize/use than the current bento centaur.
Avoid: If the creator has specifically made a bento version of that item, use that instead.
VII. MESH (bento rigged) : LL noticed that people were making items with attachment point rigging and shut it down - it's kind of a hack. But after some pushback, they decided to give creators a much better alternative: adding plenty of new bones to the regular avatar. Finger bones. Facial expression bones that react to the face sliders. Tail bones. Centaur leg bones. Wing bones! A weird singular groin bone...?
Acceptable uses: Tails, wings, hands, heads, capes and even pp. Parts that you want to animate smoothly. There's even some bento replacement parts for older avatars, like hands and tails for the Kobold and Solarian.
Avoid: Bento PP rigged to the tail bones if you want to use a tail. A good creator rigs it to the hind leg bones. Poorly-rigged/animated cash grabs - if the rare in a gacha is the ONLY bento thing in there, it's probably not a good sign. Poor-quality bento heads. There's a lot of rush to cash in, but there aren't a ton of creators who understand the technical bits of making partial joint deforms, what priority animations to use, etc.

2) DEMOS!
Always try a demo! Demo your entire outfit. Demo your body! Demo your hair. Demo your skin. All at once! Make sure you're happy with your demos. Some items, especially unrigged items, don't really need demos. If you're not sure about something, ask a friend, this thread or a group about it.
3) READ CAREFULLY.
Read the product descriptions CAREFULLY. Look at the ad CAREFULLY. Don't waste money and time buying something that won't do what you expect it to. You likely won't get your money back, and if you try, you're bothering someone who already spelled it out for you.
4) PERMISSIONS - MODIFY IS GOOD! CUSTOMIZATION GOOD!
If you want to get the most out of an item, having modify permissions is great. Most mesh bodies/heads are not modify, but you should be more discerning when it comes to clothing, armor, or parts.
Also, look for parts that have AO bakes, UV maps, or "texturing resources." These will let you customize them further with photoshop or something like that. Being able to re-use parts by retexturing them will save you money, and will make your avatar look unique.
5) BE UNIQUE, TO A POINT.
Try digging. The SL marketplace is most certainly not the  only part of SL where you can buy items or look for them. A lot of super popular products are there because they are generic: for example, the utilizator kemono has thousands of mods for it, but few people actually like the body shape or face. It's just generic and easy to use. Look through SL fashion blogs, at flickr, and at people's actual avatars to find something you love.
Of course, a lot of things are popular because they're good. There's no reason to turn your nose up at things just because they're popular - just realize that if you're not digging, you're going to look utterly generic. Buying things just because they look decently made and popular, instead of having an actual style, will land your avatar in the forgettable zone.
6) THERE'S A LOT OF STUFF INWORLD. A LOT.
Browse Flickr, SeraphimSL, and various SL blogs to get a feel of the creators out there, and their various styles. Go to their mainstores and check in to see what they've made. There's so much great stuff that isn't on the MP and if you're doing all your shopping there, you're missing out.