Over here, the law is rather vague but in theory, one can sell up to 5-7ish cars a year before being considered a trader and even then, 'they' have to prove that you're set up as such and not just someone knocking out a car every now and then as opportunites present themeselves.
Over here, things are more progressed and advanced
There is substantial case law surrounding "badges of trade"..see attachment. And as you can expect the onus is on the tax payer to prove otherwise.
HMRC is aware of the cash economy and is slowly closing in. They now monitor EBAY Sellers as well
..I suspect the introduction and digitalisation of the NOVA system will also be another tool for them to monitor imports.
Naturally each case is determined on its own merits and some folk never come under the radar. And they will ignore the old excuse,..."me mate Dave has been doin the same for years and no one bothers him! ..."
And yes I know, they should be chasing the large corporates and bankers who have millions stashed away and life isn't always fair