Know what you are buying, and ask questions. If the seller can't give you a straight answer, walk away(surf away?). Before clicking anything, figure out what you would give for this item in worse condition than what it is described in. And walk away if the bidding climbs too high. Too many people get caught up in a bidding war, and end up buying a used item for a new price.
That said, I have scored 4 sets of Conshelf regulators off of ebay. 3 in fine condition, the fourth is on the way, so I can't judge. I bought the last 3 simply to get the metal second stages, but the first stages were great, too. I paid less than $100 for all of them.
I also got a Dacor 200 reg with a bcd I bought($20 in the next town over, figured it wasn't a huge risk). I traded the reg for a pair of metal USD second stages.
I got a tank for $20($5 + $15 s/h). It just passed hydro and vip this week. But I do NOT recommend buying tanks off of ebay. I'll bid on a steel 72 if a)the price is right and b) it has been stored with pressure. But the price has to be pretty sweet for me to risk it. Plus, sellers like to jack up the shipping for tanks. Keep in mind that FedEx can ship a steel 72 for $15. Don't let them buffalo you into something much higher.
I picked up two older style jacket bcd's($20,see above, and $5). Both were in fine shape.
Finally, I got some 5mm booties recently($10.52 + $6 shipping). Look brand new and fit like a glove.
Bottom line, know the gear you are looking for and be willing to walk away from ANY "deal" if it has any hint of something weird or shady.