Last I checked one could only be taxed on what is purchased with / what you get paid for in currency.
Barter does not use currency ergo it cannot be taxed.
Said example of trading your $500 worth of labor for a $500 computer ... well, that $500 computer already paid tax by the person who purchased it with currency (the retailer, the owner bartering it to you, etc)..and your labor is precisely that: yours. The gov. can't dictate that you have to charge currency for your labor.
Or, better yet, if I grow a year's worth of vegetables and barter them for a year's worth of meat from a cattle rancher... the only thing im exchanging tax-free is my labour. Both the meat and the vegetables require land to grow on (you pay taxes for that), both require upkeep (fertilizer/cow feed, fuel, electricity, etc) which in most cases, is also taxed. So the question is... am I cheating the gov. or is the gov. cheating me when it comes to taxes?