It really depends, if you know the owner of said tyres and they are being removed due to wheel change, warranty reason then it is OK.
If buying without knowing history just at least make sure they are:
- Never punctured repair
- Less than 6yr old, ideally aim for sub 3yr
- More than 4mm across entire tyre width
- Vastly cheaper than new, half price won't cut it, I mean like vast savings.
I got some MPSS to tie me over a month or two until PS4 S is available, I got 4x MPSS for under £100, no puncture repair, 2014 tyre, fronts are 5mm and 6mm, rears are 4mm and 5mm. Got the full set for £75! The fronts were removed due to lacking Porsche approval for warranty, the rears were removed as owner changes tyres at 4mm. Just avoid puncture repairs, my reasoning is you have no idea how far they drove the car on an under-inflated tyre which considerably weakens the tyre wall and as such it could egg/blow out.
My other options was spend £1000 for the same new tyres when a replacement is due around March, £75 was worth it and having had them inspected garage confirmed they were fine, not gone hard or heat cycled too many times etc.