Even under barter and trade, there is profit. To go with the GP's example, one persona valued having his trees pruned more than he valued the time he spent upgrading the computer, and vice versa. Both parties profited and ended up better off or else they wouldn't have made the trade.

The lack of money involved just makes it potentially less efficient if the trade isn't well balanced, or potentially more efficient if they can avoid the friction of the taxman by avoiding money.