I remember buying a seconhand car once-I agreed to buy it if the tyres were replaced.On collection,I examined the tyres and was happy with them-noticed the original wheels/tyres were now on the vehice next to it on the forecourt.

I believe best practice is to have not only the same make, but exactly the same spec tyre on each axle - that is what I always do, anyway. I replace tyres in pairs.

And I've always shunned "part-worn" tyres.

When looking at a used vehicle for sale in a reputable non franchised dealership-I noticed that three of the four shiny black well polished tyres on the car were all different makes. The sales guy said that when vehicles were prepared for sale it was considered good to replace the original tyres with good quality "part worn" . I don't disagree with that-but I balk at three different makes being used. Buyers beware?

Is he really suggesting that they'd swap a 'quality' brand of tyres that are part-worn with another brand of part-worn tyres ?

Sounds like talking spherical objects to me ...

When looking at a used vehicle for sale in a reputable non franchised dealership-I noticed that three of the four shiny black well polished tyres on the car were all different make

Different makes wouldnt bother me.. I'd baulk at "dealership" and "well polished tyres".

Arguably anyone who polishes tyres needs help, but they're not getting any from me.