I was thinking similarly, what about state trademarks? I've got a Massachusetts trademark because it was quick and easy, and cost only $50. My lawyer advised me that, while not as iron clad as a federal trademark, it would tend to discourage anybody else from filing federally on the mark because they'd do a search and mine would turn up and they'd rather pick a different mark than be limited in one state. I don't sell product on Amazon though, so I don't know if it would work in that instance.