I believe, technically, any transaction involving the exchange of goods is subject to tax laws.
I know (cash) gifts can be up to a certain value before they are taxed, I don't know if it is similar with items (you pay tax on the purchase of a car, not to give it as a gift? though giving that much money as a gift would require some government involvement, iirc).
The IRS basically sets what is in its own jurisdiction, so my assumption would be that should bartering start to impact the state's flow of capital, to turn a phrase, the mafia would demand its cut.
Yeah, I just talked to a tax professional, because I was curious. (He works in another department here, with me.)
He said basically what inimalist said. Just the same as someone buying something at Wal-Mart, each party pays taxes on their respective "goods".
Except, instead of one receiving money and one receiving goods, they both receive goods. (Income taxes, state taxes, excise taxes, would all still apply.)
Actually, considering it'll probably be legalized soon anyway and looking at the great depression and bootlegging and families like the Kennedys, growing and selling pot may historically be the best thing to do right now. I'm not encouraging law breaking, I'm just saying that based on the history of the late 20's and early 30's, that loser in the smelly hoody with the lame chin strap beard may have a mansion in 5 years and a son who grows up to be president.