There are a few questions you have to ask yourself - why were they removed?
Usual reasons:
Car scrapped (possibly due to accident), so may have been some impact to tyre
Flatspots/run out of true
Owners had a non-repairable puncture, and wanted matched pair
Tyre getting old
Additionally, remember:
History of tyre is unknown, with may have sustained non visible impact
Check the date of tyre (on sidewall)
New tyres are 9mm, and really need to be changed at approx 3mm. Although that makes 6mm the mid-point, the first 3mm last longer than the next 3mm - for this reason, always using part worns is false economy
However, its little different to buying a 2nd hand car, and using the tyres it came with.
Personally, I wouldn't consider part worns, but with money being tight for everyone, its the only option for some... ...and part worns are likely to be safer than bald tyres.