Bartering is generally taxable.

Whether doing business with bitcoin is 'doing business with money' or 'just bartering' is irrelevant, it is still considered as generating income.

The value is the the 'fair market value' of goods and services.

So, even if you sell a house for 10 dollar, the government can still value it at market value and tax according.

It is similar going through customs. You have to declare the things that you are bringing into the country.

If you bought an US$20,000 expensive Rolex, you can say it is for personal use, you can say you bought it for US$20, but the customs officer makes the final judgment. Even if you really bought it for US$20, the customs officer can still tax you based on the market value of US$20,000.

As for doing business in bitcoin, whether you have to pay taxes depends how much you earned. If the total income is under minimum taxable income, that's another issue.

I guess most people are using bitcoin as a hobby at the moment, and the bitcoin economy is really small, so the government won't bother to look into it. Or the government haven't even heard of the term bitcoin.