There is no scam in offering to buy something if you follow through and pay what you´ve promised to pay. Most users whom now have made several dollars on selling their STR would otherwise  not have known about the giveaway, and thus made exactly zero dollars. Making several dollars for just a few minutes of simple clicking is in any way a good deal.
But I agree, people are really making serious efforts to aquire STR. Here´s one person buying stashes of stellars for two dollar: