It depends on the custom - primarily depending on if the character is an infringement of copyright. If you were to kitbash a schoolbus that transformed into a robot using bits of models and action figures (e.g. Gundam, Transformers etc) then it's not a breech of copyright afaik. But if you were to make and sell say Masterpiece Swoop, then yes, that is a breech of copyright because that character is a copyright trademark (and that includes name and likeness).

Years ago ToyFare magazine published an article where eBay staff stated that such customs were illegal and that such action would be taken if such auctions are brought to the attention of eBay staff - I imagine that they usually just cancel the auction.

But as dirge said, I don't think it's something that Hasbro is specifically targetting.