I'm guessing Costco would be a bit reluctant to formally allow something like this, as I also presume that one reason they get lower prices from suppliers is that they only sell to members, so there is less of a perception that a supplier is undercutting other retailers by letting Costco sell at a reduced price. A sort of price discrimination... but if stuff gets resold without a markup to bring the price in line with other retailers it would make their negotiations more difficult.