I've often asked sellers what they want for a bike, and when it is too low, I've told them:

"Look, your bike is worth x. I can pay you y. You can probably get x, but here is what you would have to do (insert steps of ebay sale here)".

This is how I treated the purchase of an MX Leader I found at a yard sale. On another purchase, the seller basically gave me a box of Suntour superbe for being honest. The MX Leader seller said one of the reasons he sold me the bike is I was the only person he talked to about it that was honest about the bike. More than one person told him it was worth 50$.

I'll admit there's been a couple of other cases where I just got the money out once I heard the asking price. Obviously, this is a bit of a thorny issue. One of our advantages-- and the advantages of good pickers everywhere-- is we have a bit of specialized knowledge very few people have. I see plenty of bad pickers whose knowledge stops with their smart phone. I suppose that our education in matters bicycle, we should get a bit of a premium now and again.