There's an admittedly fine line, with in-game "property" sold on ebay when most players are just wanting to play the game... but with bitcoin it's nowhere near that fine line, particularly when you're engaging in manifestly economic activity like the above with real-world things.
Edit: I stand corrected by the comments below.
I think the rules for commodities and currencies are likely different. Regardless, bitcoin is clearly one of those two and you don't get to avoid the IRS by only dealing in commodities.
(Note: this is known as a "method of accounting" in tax parlance.)
I'm not completely familiar with how it functions, although you can look up more details [1]. I would not be surprised if in any case that rancher Joe decides to trade 2 head of cattle for farmer Bill's old tractor, there is a tax obligation that needs to be paid. Of course, this probably doesn't actually get paid very often.
1: http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employ...