It is standard practise to provide at least some road tax so that the purchaser can drive the vehicle away instantly.
However, I don't believe the vendor is under any obligation to sell the vehicle with tax. They may have an obligation under a contract of sale or some other document if for example, they undertake to tax the vehicle in their terms and conditions.
If the logbook is transferred into your name at the point of sale (which will almost certainly be the case), you become the keeper at that point and are therefore committing an offence by using the vehicle untaxed on a public road. That's why it's unusual to sell untaxed vehicles, especially on a Sunday when there is no Post Office open to tax it at.
It is you, however, who would be committing the offence by allowing your untaxed vehicle to be driven, and not the dealer.