Honestly HATE it, like you put it up for sale/trade for a reason, I personally would avoid people with a mindset like this like the plague. Once traded/sold away its no longer yours, the only right you have is the right to the artwork itself NOT the character, if you wanted to keep it you should have at least thats what I think they should have done.
Its way the only rule I really have is like don't resell/sell it more than you bought it for unless extra art was inculded. Yk the typical rules, I already know i lost ownership of the design once it went to someone else. Like its just a very bad mindset in general. As you said it ruins the fun.