Depends on how many laps they've done and how good you are!!!
I've bought a few pairs in the past and had no probs, they're usually ones that have done a race with a team and they want to start the next race with a new pair.
Unless your racing seriously, you should be fine with them - and pay around £100 the pair???
ive had part worns fitted at donington,allways a chap their when trackdays run, been great,no problems.the compound is soft (sc1) etc.they dont last to long but who cares
I've bought part worn 208Gp's before, some have been great some have been so so, looking back I could see the wear on some more than others, and the lifespan etc was affected also.
I picked up 2 rears the other week and one has seen a bit more action than the other, but they'll still last me a good few track days next year
I bought some SuperCorsa's off weeksy. I got through 1 wet track day, a dry track day and a CSS level 1 before they were looking really bad.
Having said that they were still quite grippy yesterday at Rocky but the rear has lost it's profile on the right hand side and seems to be pulling apart. I blame the chicane leading on to the straight at Silverstone and the hairpin leading onto the straight at Rocky.