Someone probably put in a false "complaint" (I won't speculate as to why!) and the ignorant twits at eBay rushed to withdraw your "assault rifle parts", as they love messing firearms people around if they can and wouldn't know an assault rifle from a BB gun if their lives depended on it.

Anyway, if you want to relist it, just do so with a big red notice in the description that your scope is not an "assault rifle part" and complies with "eBay listing policies". That usually shuts them up.