You're assuming someone would be dishonest with the badge when they bought it. I would not assume that, but the more likely scenario that they'd use it on a period-correct drum to complete a set or something. The dishonest people can usually be found out by a discerning eye.
It could be construed as dishonest to part a drum out and sell it, but it is, as John said, an economic decision to do so. Being dishonest on the other hand, I've found, is in the long run a poor economic decision.
Anyway you're selling something, and your honesty is more of an issue than someone else's. If, on the other hand, you were selling a guide to pickpocketing or something like that, there could be ethical issues. Just selling a drum part isn't an invitation for the buyer to be dishonest.