Hate to break it to you guys, but it's perfectly legal to buy and sell goods with private currencies. Laws have been on the books for the past 100+ years to allow you to do this. The only problem starts to occur when you do it regularly that the government wants to make sure you count it as income. Sure , that sucks, but if you want to be above board and perfectly legal you can do it.
The form the IRS issues for non USD transactions inside the US is the
1099-b and you're required to use it for transactions on a yearly basis over $600 or so. There is one brokerage out there that has over 50 million dollars of goods (Services & physical goods) for sale. No government clamping down, no issues.
The reason everyone and their brother says that it is illegal in the US is due to the fiasco with the liberty currency. And as someone who trades/buys silver I can fully understand the government's reasoning for shutting them down. I don't agree with the asset seizure, but it was quite obvious for anyone overly knowledgable about US currency that they were doing something borderline illegal by issuing currency near-identical to official US currency in the eyes of the general public.