Ragamuffins

Hello! I am just looking for some advice on how to proceed, or if any one thinks this warrants a beware post. I am having issues with a client that purchased two adopts from my recent sale of characters, and they also ended up purchasing two more of my former characters from some one else I sold them too. The main issue that I have is the client in question has been messaging multiple people in the past month attempting to resell the characters at exorbitantly increased prices. 5-10 times the original sales price. I have notes and email records from multiple people that have since contacted me about them with their attempting to resell at inflated prices. I don't mind if people resell my characters, and would understand a markup if there was considerable artwork commissioned of them in the time that they owned them. But this is going from the $50 that they purchased them at, to an asking price of $500. Unfortunately I did not have any TOS down about restrictions reselling the characters, so I am limited in that regard. But would it be too much for me to refund them the amount they paid to reclaim the characters? I can't do anything about the ones they bought from the other client, and I am trying to reach them to see what they sold for, but it really makes me a little sick to my stomach that they are marking them up SO much. Any advice would be super appreciated! Thank you!