Buying Used Equipment on the Internet

Hello all, 

Had a question for the more experienced hi-fi enthusiasts here. I'm new to this hobby and currently still trying out new gear to see what I like. I was looking into trying out the Parasound Halo Hint 6 amplifier and found a used one through AudioMart. 

My question is: what advice would people have for buying used equipment online? The pricing is attractive but there's also no option for returns once the transaction goes through. What's to stop someone from offloading a lemon on to an unsuspecting buyer?

Thanks!