I know it's an auctioneers ploy, but why treat their prospective purchasers as idiots, as any prospective collector/purchaser will know an item's true value.
As a prospective purchaser, you would like the item advertised to be known about by as few people as possible, whereas if you are a seller, just the opposite applies, so mentioning the item on the BBS may well benefit the seller, but with the internet as developed as it is, there is little chance of an auction item going unmentioned.