Dangers of buying a used iMAC

I am considering the purchase of a 2011-2012 27" iMac, 2.7Ghz, 8 gigs ram, 1 TB hard drive, wireless keyboard and mouse for $650. But I am wary. I wonder if there is a known scam whereby sellers of used computers install keyloggers or similar software which they can use to steal the personal information of the buyer, me. I'm told I can try out the iMac, but how can I tell if such stealth software has been installed? Does this sort of thing really happen?

Looking for advice.

Thank you.