Generally I don't have concerns to buy used computers, I never had a failure and my mini is up and running since 5 years (-3 days to be precise, MorphOS 2.4 got released on 12 October 2009) now.
Yeah, I haven't had problems with used computers either. Only problem was that year ago one of my a1200s died, but it was a find which was stored on outside in the rain in pile of junk and it had got rusty all over, but still it worked fine for years

But anyway, my newest computer is Powerbook which is 8 years old now. My old Peg1 (bought used) and even classic Amigas work still. And my newest PC is from 2004 and working fine too. Mac mini has been the daily machine since release of the MorphOS for it and still going strong.