The truth of the matter is that most people don't report these sales, so most don't pay tax on them. If we all kept receipts for what we spent most of us would show losses. - Or very little gain.
For almost everybody here this is a hobby. We buy used parts from all over the country, buy the materials we need to recondition them and eventually move on to the next project. If we were to track every cent spent on gas, shipping, paints, electricity, storage, etc... we'd see that we're amongst the worst businessmen ever. Often we'll have to buy 2 or 3 vehicles to make one. Heck, even when we do our own labor we can end up upside-down on our investment.
Legally I suppose we should report the money changing hands. But as is we struggle to be able to afford what we do. If the government was to seriously crack down on all these transactions the outcome would be to shut down work at home hobbies. And hobbies like ours support a lot of other businesses that generate tax revenues. I'm sure that some may be reporting these sales. But most likely the cash obtained will be re-invested in the next vehicle. And so there is no actual profit.