i have mixed feelings about this! I feel that there are appropriate ways to trade/resell art and inappropriate ones. it's also very different when comparing custom and premade art. overall it's way too easy for people to exploit.
for example; i personally have a few oc's. i buy art for them, draw them, the works. but i have, in the past, realized i dislike one of the designs and just let this beautiful art sit in my sta.sh. i gave the entire character design and any art of the character to a close friend. i made sure all the artists knew and agreed to it. if a buyer reached out to me and asked if they could have permission to trade away the art i drew and were also trading the oc design, id be okay w it. That, in my eyes, is just re homing the art and design with no ill intent. if someone wanted to resell/trade my art of their oc but was going to keep the design of the character i would refuse. if they wanted to resell my art in general, id refuse. that seems like a quick way to gather new art and an easy way to get money.
i dont do premades so i dont fully understand how premade artists might feel. heres my two cents though. If you arent happy enough with the piece to keep it forever, dont buy it. Dont buy premades with the intent to trade them like pokemon cards. if you used the premade for 2 years and want a change, i suppose trading it away with permission for a different premade isn't horrible. if someone is trying to resell, that 100% seems like a cash grab. even if they resell it for 10-20 dollars, thats 20 dollars they wouldnt have had before.
i will never give a buyer permission to sell my art. i will never give permission for a buyer to trade my art as a form of currency (i.e. "ill give you this drawing if you draw my oc")
tldr; if you want to resell/trade art get the artists permission. reselling art 100% seems like an easy way to exploit artists. if youre going to trade custom art, you must also trade the character. if youre going to trade a premade, you may only trade it for a different premade.