To prevent cheating when trading with other players, you must pay a minimum of 2 gold per item in pre-hardmode, a minimum of 5 gold per item in hardmode, a minimum of 10 gold per item post-plantera, and a minimum of 20 gold per item post-lunatic cultist. Gear obtained from bosses, golden chests in underground cabins, etc. may not be used and must be sold to an NPC or another player as soon as possible. If you intend to buy the gear back from the player you sold it to, which would allow you to use it since it would become gear that was obtained through trading, you must pay at least twice the price you sold it for. For example, if you kill the eye of cthulhu and get a shield of cthulhu, you may not use it since it's a piece of gear you did not buy/trade for. You decide to sell it to a player for 2 gold. You may then buy it back for 4 or more gold. You may use it from that point forward since it became an item that you bought/traded for.