I form an overall impression of a seller from how many suspect coins I see in their listings. This helps me decide whether to take a chance on the one I'm interested in. If there are several that look like fakes, I pass on them all.
Another red flag is in the seller's description of every lot:
"If for any reason you feel like you may be unhappy with buying this coin or feel like you might complain after receiving it please refrain from bidding."
which to me means:
"if you are going to be upset that you bought a fake and were ripped off, then don't bid."