$350 is a great price for a minty M-520, but again like has been said test it out. Take your time and come prepared to test it and if he pressures you be suspicious.
Bring a couple sets RCA cables for cross patching stuff to test, a phantom powered mic if you've got one handy, a mic cable for the mic and one to test the balance amp outs, set of headphones...you can pretty much test the whole board with those things with the exception of the instrument input (bring a guitar and a guitar cable to check that out if you like) and phono amp...need a turntable to do that.
Bare minimum test each mic input and make sure you can get it to each subgroup and to the headphones, and then you can quick test each line input on each channel by hooking the mic to a mic pre and then sending the direct out from that channel to each line input one at a time using the RCA cables you bring. Then test the balance amp section by using any RCA output into each balance amp RCA input one at a time, and then connect each balance amp output one at a time to a mic input (phantom power OFF) with the -30dB PAD inserted (switch to the right).
Good luck!