Make sure it plays OK - in tune with a REALLY low (in comparison to a guitar) action all the way up the neck and no buzzes. It's much harder to get a mandolin action sorted than a guitar as the clearances are SO small. Lots of cheap mandolins have crap intonation too.

If it plays OK and the intonation is OK (get an expert friend to check, most guitarists say mandolin action is OK when it is WAY too high: all things are relative) you can get one witha better sound later. If it plays like crap you will never want to play it.