Ever...with all due respect, I think you're taking this far too seriously. This isn't Wallstreet and we're not negotiating the end of a war. We're buying and selling bikes, primarily cheap bikes, from a drive way. As others have said...if you;re approaching this as a way to make a living, you're in trouble regardless of sales and purchase prices.
Sales man psychology? This isn't a $5,000 purchase you have to negotiate off the wife. For the most part, people are reasonably educated buyers and sellers and have a general idea of something's worth. Not always, but usually. If something is an incredibly good deal, why bother trying to skin out an extra $10 anyway? You go after the ones that you know you can make a reasonable profit from, and you avoid the ones that are incredibly over priced. If they;re insanely over priced, they won't sell and the person will eventually figure out that he needs to lower his asking price anyway.
One question that I have involves people talking about purchasing at flea markets...the flea markets here (Philly metro area) ALWAYS sell pure junk at too high a price...like $75 Magmas. If they do have something nice, they always know and price accordingly. In my experience, flea marketers research the value of things before they sell them.