There is a big difference between Used, and "ex demo amp". An ex demo amp is usually fairly well taken care of, screw heads not stripped or chewed up. I say "usually" because you don't know what gauge wiring they used or how the gains were set. I have seen some amps ran on 2 - 4 gauge that should've been on 0/2 and gains set by ear.
Personally, I would feel confident in that amp, especially with the 6-month warranty they're offering. Used amps are a crapshoot unless the seller is reputable. I've even had 'reputable' sellers ship junk that wasn't as pictured.//content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif