Is it legal for a motor trader to sell a motor home with tires over 6 years old?

If the tyres are in good condition ( treads on tyre and have no punctures )

It would be fine :)

Im guessing the Motor home had been left standing for a while so make sure you pump up the tyres to the correct Psi Specified with the model, also check the pressures over a 3 day period for any dramatic change.