Buying UC pets is a process. Does no one read the after purchase "care" guide?
Edit: your reply to Mod makes ya seem competent.
Because the pet in the OP's scenario moved, it's possible a previous owner proved ownership to TNT. So a trade reversal happened. The OP hadn't traded, so no pet was returned there. Shell was frozen for being the middleman.
I'd contact the seller to see if he knew why this might've happened. If it's more likely a shady seller than an inexperienced buyer, I'd want some form of compensation/justice.