I don't know any of the members here, so I won't assume or imply that the person you describe is this way.
However it does sound a little like someone holding a card hostage for the sake of a lopsided deal in their favor. I think most player or set collectors have probably run into this at some point. You want a certain card of a certain player and inquire (showing your interest). Suddenly that player or particular card is the greatest, most hard to find gem in the world and you won't get it for less than 1 arm AND one leg!
I am not a set builder and have no real love for common cards in general, with one exception, if they are Dodger cards. In this scenario, I'd trade you a common for a HOF in a heartbeat, all other factors aside.
PM me what set you are talking about. Maybe I have a common you need!!
