Like it or not -- especially for used gear -- Ebay sets the market price. Antiques are a little bit different as each piece is unique. So if a buyer wants to buy that one piece, you might get someone who gets carried away.
But if you're looking for a lens, say a 10-22 in good shape, there'll be an auction ending today, and two more tomorrow and four this weekend. Why would anyone be willing to pay significantly more for the one today than for the one tomorrow -- or vice versa? If you look at the 'Competed Items' listings, you'll see that everything goes for within a few percentage points of the average.
I bid low on a lot of stuff in the hopes that I might "steal" something, but it never happens. If I'm really looking to buy something at market price, I go figure out what that is and bid accordingly, trying to pay no more than the average.
If I'm looking to buy something used someplace other than Ebay -- say from the classifieds on a forum or off Craigslist -- I still go to Ebay to figure out the market price. Sometimes a seller will say, "Don't go to Ebay. The prices there don't matter because..." I've never heard anything that makes sense after the "because." Every day millions of transactions close on Ebay. And when that gavel falls, the value of that particular item, as determined by the dozens or hundreds of people who considered buying it, is set.
I'm a fly fisherman and I buy a lot of used fishing stuff. High end fly gear in good condition usually sells for about 60-70% of new. I'm just now getting back into photography. (I was a pro in a past life, but I'm new to this whole digital thing.) I was surprised to find that the market for used camera gear is more like 85-90% of new. There are a lot of advantages to buying new -- a warranty in your name, manufacturer support, retailer support, no hidden surprises, etc. That all adds value. It surprises me that people would trade that value for $75 off a $600 lens. But that's the market. If I want to play, I have to play by market rules.