Street dealers have untaxed/uninsured cars

forgotmyname

Not that hard to detect some traders, They accept part exchanges where most private motorists wont.
The same phone number for car after car and as soon as ones sold another one gets listed.

And lots more that the silly ones dont think about. Like making a nice profit on a particular car then buying the same model over and over.

rev_henry

It is possible they have a traders insurance policy which won't show up on MID - will just cover all cars connected with their business. Certainly shouldn't be advertising as private though and should have tax, unless they have trade plates on whilst on the public road.

If the cars aren't registered to them (or possibly are shown as 'in trade' by DVLA) there's no one to send letters to for no tax.
Having said this not all driveway dealers are dodgy, it is possible to be perfectly legitimate.