Do gambling laws in the US really not cover dollar auctions? For some reason I always thought such laws would be why every promotional sweepstakes in the State

Do gambling laws in the US really not cover dollar auctions? For some reason I always thought such laws would be why every promotional sweepstakes in the States, e.g. McDonald's Monopoly or Coke bottle codes, etc. have a rule in fine print about "No purchase necessary to enter. To enter, send a self addressed envelope to... " etc. so it could be argued the sweepstakes is not pay-to-enter. Even then, such promotionals usually have further notes about not being able to enter in three or four out of the fifty states. It would seem regulation would be desirable for such auction sites, since the consumer has to place a significant about of trust in the auctioneer not to bid up his own products to make more money, or throw non-profitable auctions to a self-owned account... the end-user currently has no practical recourse if this happens. Hopefully grellas or another IAAL can throw in some knowledge.