A couple of months ago, I did an auction of several character adoptables of an original species of my creation. Within the rules, posted very clearly on the FA description of the auction right above the instructions for bidding, I had a single rule. No adoptables purchased from me may be resold for more than they were purchased for. I added this clause because I had seen people purchasing adoptables for cheap and then turning around to resell them and profit off of an artist's work they put no effort into, but I wanted people to still be able to get their money back from it if one day they decided they didn't want the character.
One user purchased two of my adoptables and now I see they have hoarded many adoptables to resell and are selling both designs for more than they purchased, one of them for almost double what they paid me. I am also not credited on either of these adoptables being put up for sale, nor is the species name listed, so if someone purchases them, they won't even know I made them to begin with.
Do I contact them and remind them of the rules and ask them to take it down or lower the prices? Or do I just let them know they're banned from all future adopts I sell because they broke the rules? Is this a common problem for people, rules being broken on their design purchases?
Update: The problem has been resolved without any bad feelings and I've been credited in the sale. So, no measures had to be taken aside from messaging them and I don't plan on banning them from future sales since it was apparently just a mixup on the way they organized the adopts and their prices. Which is a huge relief for me since it does mean they were just buying adopts from people then later decided they needed the money more than they needed all the characters, no maliciousness intended there.