Ben wants to sell his used couch to pam. ben and pam will trade the couch for money, if both individuals expect to

The exchange will only happen if both indivuduals want it, that is, of both of them think it will be profitable to them. In other words, it will only happen if both indivuduals expect to gain from it ("gain' is the best word for the blank").
(Sometimes people are forced to unprofitable exchanges, but this doesn't happen in a free economy)