Transactions are just gifts bound together in pairs like the atoms of nitrogen in the air. Just as nitrogen is inert when the atoms are paired, transactions are tremendously useful compared to other gift giving customs. We give away our money to strangers because we have the expectation that these strangers will then give us merchandise. Without this pairing we would have to individually know and trust everyone we dealt with. Only in small groups like families and tribes can this be the case.
Gifts also provide a way to extend the idea of a free market to primitive cultures.
Sometimes these societies don't have anything we could really call a market economy but are instead permeated with elaborate gift giving rituals. A citizen's status and whole life may be determined by the gifts they give. Where gift giving exists in any culture personal property exists there also. Only what is owned can be given, only by the owner, and the receiver becomes the new owner. Many view ownership as evil and greedy, but who abolishes ownership also abolishes gift giving.