Not necessarily specifically Porsche advice but I do think the "used" part mentioned by M40WST is an important one. 10k miles on a 10-year-old car may sound good but could be disastrous if the last 8 of those were sat unused in a damp garage. Equally, 60k miles for 6 years old may sound like a lot, but may be absolutely fine if serviced properly and well-looked after, particularly if they're mainly motorway miles. Then there's age itself (which comes to us all). My 2005 Boxster has just crossed 50k (and is about to go in for a service), so unless I go insane mileage-wise it'll still be way below any kind of average when we trade her hopefully at some point later this year but, she's is 15 fifteen years old and there are bound to be some bits and pieces that have simply aged regardless of servicing and care.
When we trade I'm hoping for a 987.2, 10 years old max (although you can't get much older), with no more than 40k on the clock, used regularly every weekend but not commuted. So, pretty easy then.