I already said that this arrangement takes place in your hypothetical society, so there is no position of power. Bob can work elsewhere, but he chose to work for Alice instead. Your ideology cannot handle two people making a voluntary agreement without labeling it as exploitative, so I think we're not going to be able to have any further rational discussion.
Sorry, I misread. In the case of a cooperative society, if Bob gives Alice the product of his labor in exchange for anything less than she might turn around and sell it for, the he is either foolish (and Alice is taking advantage of his foolishness), a fetishist, or doing Alice a favor.