Buying used will always have it's risks. Trust your seller, ask plenty of questions over what's not disclosed (like tobacco or musty odors, etc). If & when you feel confident with the offering, pull the trigger. Most of us buy used gear all the time & mics are on the top of the list (preamps are surely a tight second place). Most times great deals can be had, now & again a bum sale comes your way. The good deals always outweigh the bad ones, so the sour grapes come out in the wash & hopefully you learn something from them. Most times both buyer and seller walk away happy. Good Luck in your hunting
Oh, and whatever you do avoid eBay seller "9guitar" a.k.a "Flower Pot Guy" (he's a reckless price gouger!!).