Buying a used string instrument, what does one need to know?

I'm saving up for another ukulele, or maybe a guitar, or possibly a mandolin.   Haven't decided yet and I'm a long way aways from buying.

Someone offered to take me around the music shops next week so I can get a better idea of what is available in town.  Then I can save up for when I'm ready.  

It's exciting, but also scary.

I went to a pawn shop recently and the strings instruments were either badly broken or obviously never played.  And when I looked the ones I remembered later that day, the used ones were all about 30% more expencive than the new.  Even the mandolin with the concave top was $100 more than the same make and model new.  Even I know a mandolin is not supposed to cave in at he bridge.

I'm hopefully next weeks expierence will be more pleasant.   I think it would go better if I knew what to look for and dint look like a completely new to instruments.

What does one look for when buying a used acoustic string instrument?

Depends on the instrument. For example a cello has a much taller bridge than a violin, and under higher string tension, and so bridge warping is more of an issue to look out for on cellos than on violins. With mandolins and guitars you have frets to deal with—are they cut properly, are they worn, etc. Tuners on a classical guitar operate differently and may have different issues from tuners on a folk guitar.

So in the absence of more detailed information, I would say play the instrument, making sure your left hand is feeling up and down the neck, paying attention to how it sounds and feels. Consider things like whether it’s intonated properly, whether there are any “dead” notes, and whether it ever feels awkward or like your hand is getting scratched by something, etc.

Also keep in mind—this is more true for electric string instruments but still mostly true of acoustic instruments—everything is fixable.

Tuners on a classical guitar operate differently and may have different issues from tuners on a folk guitar.

Could you tell me more about this?  I know some guitars aren't meant for steel core strings, but I didn't know the tuners were different (except the ones with the cut out head).  Is it related to string type and the big rod in the neck?

I learned on my recent ukulele setup experiment that I perfer geared tuners to ones that stick out the back and have a one to one ratio of turn to tune.  Like shiny violin pegs but smaller so they are more fussy.