if your reading adverts for stuff it's likely on ebay, threfore the 1st rule of buying 2nd hand things online applys.
if there is any possoblity that it can be knackerd it will be knackerd. ebay is basicly like dial a pikey. bung scrap on there and pikeys collect it.
it all depends where te part worns came from

would you turn down a set of 4 continentals from a car that left the showroom that week and only had delivery miles.
most part but not very worns i fit have 6mm tread and decent brands