Have you discovered that if you have something that someone else wants, and they have something you want, and you swap, then you can both end up happier? Have

Have you discovered that if you have something that someone else wants, and they have something you want, and you swap, then you can both end up happier? Have you discovered that it's tricky to carry with you for exchange purposes every conceivable desire of the guy that owns the grocery store, and thought that it might be handy to introduce an abstraction layer?