Two weeks ago I traded two beat to death DX discs at PIAS for an almost new Star Archon that still had the sticker on the rim. The girl behind the counter had no idea what I was trading in, just said $2.50 a disc. Who knows, she may have priced the Archon too.
It's not my job to inform the seller, it's not the sellers job to inform me.
I've paid a few bucks too much for a disc on impulse or because it was exactly what I wanted. There is something to say for being able to feel a disc, get the right dome, weight, color and plastic.
I've also saved a few bucks on discs I mail ordered and ended up not like them because I couldn't feel them. Had I been able to sample before I bought, I never would have purchased them.
Six of one, half dozen another.
The worst I have seen though is a place selling MVP for $22-$25 a disc! sorry, not doing that, no matter what the impulse.