I. Please notice
I've heard several people saying that they are having issues with Master Merchant (we'll call it MM from now on) or Arkadius' Trade Tools (ATT). It could be that you are using one of them for the first time or that your addon is working as intended but the limitations implemented by ZOS make it look like they're not working. If you are looking for some aid then here is a small guide that could really help you!

Please always make sure to have the latest update available and to run the libraries required for the addon (applicable to every addon). As far as I know ATT and MM do not require any extra library currently (this could be false in the future). I have never used Minion so I won't be able to help you with it.
Both addons constantly and automatically gather data from your guilds but this can take from a minute (if you your data is fresh) to a few hours per guild (if you need a full scan)! Thankfully this guide includes a way to speed things up.

Very important: Your addons do not gather data when you're offline (obviously). This is why you need to update your data everytime you log on after a 'long' offline period (several hours or more). Either let your addon do it automatically but this might fail if you don't stay online long enough or have been offline for days. I advise IV. Gathering data manually to keep your data up to date.

II. Master Merchant
Before you check any of the following situations you could be in, here is an important thing you need to remember : when MM is scanning a guild, all the sales being scanned will not show until the full scan is completed. So if your MM has been scanning and you can't see new data it's completely normal: MM is doing its thing.

Case 1 - You've just (re-)installed MM: Upon login MM will scan your guild sales to gather data. You can let it do it automatically or speed up the process with the Manual Method (See IV. Gathering data manually). Please keep in mind that MM will display data in the main window once the scan is over so if you can't see anything and MM is not done scanning it's completely normal!
Case 2 - You've just joined a new guild & MM is scanning that new guild: Simply check IV. Gathering data manually if you wish to make the scanning faster
Case 3 - You've just joined a new guild & MM is not scanning: Now it could either be that MM is working as intended or that it's completely bugged (seems to be a recent issue). At first try to relog. Back in-game: MM will scan. If it takes a while and/or scans your new guild then MM is working and either IV.Gathering data manually or let it scan automatically (this can take a while). If it stops scanning after 2-3 mins without getting any data from the new guild then I would assume your MM is bugged and you need to Exit the game->delete your MM Saved Variables->Log back in->Let it scan everything or speed up the process with IV. Gathering data manually (you're basically in Case 1)
Case 4- You keep your data updated but MM is still missing/losing data: MM tends to lose a bit of data from time to time so you might need to run a /mm missing. In the chat type '/mm missing' (this will check the missing sales for all your guilds in the last 10 days) or to be more specific '/mm missing x y' where x is the guild you wanna scan (your first guild is '1', your second '2' and so on...) and y the amount of hours you want to re-scan (10 will scan the last 10 hours for example). Let MM get the data itself or speed up the scan with IV. Gathering data manually

III. Arkadius' Trade Tools
ATT is great in that I've hardly ever lost data while using it and it does not bug. It does not matter if you've just installed the addon, if you've just joined a guild or if you simply want to gather data like you do every day: ATT will scan regardless and you can check IV. Gathering data manually if you want to speed up the process.

IV. Gathering data manually
1. Open the in-game Guild Menu (the guild you want to collect data from first) -> Open the "History" tab -> Click on "Sales" -> "All"
2. Spam the "Show more" button at the bottom (not too fast otherwise a crash is awaiting you, once every 2 seconds should be perfect)
3. You can scroll down in the log/sales and see that there is a time for each sale ('x hours ago') and this is the important part: if you need all the data possible then you will have to spam until you can't anymore (the keybind at the bottom will disappear). You should have reached '10 days ago' for the oldest sale then. If you only need to gather for x amount of time then no need to go through every single sale, just reach 'x hours/days ago' and you're done!
4. Let your addon breath for like a minute so it can gather everything!
5. Repeat for each guild you need data from

V. Additionnal help
Below you can find the folders containing my own data from our 5 guilds (the files go into the SavedVariables folder). I will try to keep them updated but I can't repost every day. Just see them as an easy way to get data when you start with nothing but of course I still recommend following the comments above to get your own data. If you do use these folders you must check your ATT/MM addon settings when you log back in because it will add my own settings and will therefore remove yours. If getting my data causes any issue for your game please let me know ASAP. I've done it in the past and it went well but my knowledge is not big enough to guarantee this will work every time and for every update.
If you have more questions or if you would like to correct me on something, feel free to post in the #addons channel or to message me!
If you have more problems with those addons and we can't find a solution feel free to visit the esoui.com comment section of each one.