A corollary to ChosenUndeaad, artists, LEARN WHAT A LICENSING AGREEMENT IS, LEARN HOW THEY WORK, and start USING THEM IN THEIR LEGAL SENSE.
If I buy your Very Popular Adoptable(TM) and there’s just a verbal “Dun sell it for more than I sold it to you! WAAAH!” on your sales page/site, yes we CAN sell it for more than you sold it because you didn’t have a LICENSING AGREEMENT UP.
For your sake, learn about them and use them.
To play devil artist’s advocate, though... If someone breaks an artist rule of yours but you DIDN’T have a legal licensing agreement up, you may have no LEGAL recourse against that person but DEFINITELY make a “Banned Buyer” list and add them to that list.
It may be LEGAL to flip adoptables for higher prices but if the creator asked you NOT TO, it’s kinda dickish to do it anyway.
And for those select few creators out there who ARE deliberate twits, just because you think it sounds more expensive and cool to tell a buyer he now OWNS the art he bought from you, but really, he needs to follow your petty rules, then you deserve whatever vexatious nonsense you get for this stupid thinking, you twit.
TL;DR Licensing agreement means you can enforce your art “rules”. NO licensing agreement means you CAN’T. DELIBERATELY choosing no licensing agreement but then throwing a fit when your art “rules” are broken makes you a petty twit.