One of the difficulties of separating small-tribe economics from the rest of their social system is that the members of a small tribe don't think of it like that. If you live in that kind of society then you know personally the people you are dealing with, and while you might "trade" items or favours, its all pretty approximate, with the "balances" held in peoples heads, and tied in with other concepts of social obligation such as a general duty to look after the sick and elderly, and watch out for each other's children. It's only once people start living in larger groups where you can't know everyone that something like an economic system develops.
