I've pretended to come back from long hiatus on an account I bought before. It's kinda even believable right now because the Jumpstart thing HAS made a lot of people come back just to see what's up.
Just pretend general confusion, ask some dumb questions about how to play, then proceed with trading your pet and nobody will question you.
Gonna be harder to convince people you came back from hiatus and don't know how to play but have millions of items to sell, though