It is not trademarked. I don't know about the legal theory but at this point if someone tried to trademark it it would likely get denied as being in the public domain.

You can use the name bitcoin to sell whatever you want. Trademark laws won't prevent you from selling fake gold coins and calling it bitcoin but there are consumer protection laws that would give buyers recourse and likely Amazon would pull them and ban you. I suppose if the fraud got large enough the state could charge you.