If you do not know how to play saxophone, you may be at a disadvantage when purchasing a horn, especially a used one. Saxophones are not all the same; each has a unique set of intonation and timbre problems. The best way to purchase a saxophone is to bring someone who knows how to play it and have them try it out. If you are a player yourself, it is easy to get caught in the moment and not try a horn out as thoroughly as possible. I am as guilty of this as anyone, I am afraid. I once purchased a Selmer Mark VI soprano in seemingly beautiful condition for a good price. A week later I realized that the bell was physically cracked on the inside seam. The repair would be unsightly and expensive and it was too late for a return with the dealer who sold it to me.
I am also reminded of a curved soprano I once found in a pawn shop. It was an early Conn in mint condition with gold plated keys and bell. The saxophone looked brand new, and the pawn shop was asking a reasonably low price. I played it and things seemed basically okay until I got the tuner out of my bag. The horn played almost a quarter tone sharp on some notes, and quite flat on others. I tried to adjust and use different mouthpieces, but no combination seemed to affect the intonation of the horn. I finally gave up and thanked the owner for letting me try it. Had I jumped on the horn because of its looks and condition, I would have been stuck with an unplayable soprano. I figured that the intonation problems may have been the reason that the horn looked so unplayed. Some "closet" horns out there are great saxophones, but keep in mind that ugly horns can play just as good or better. How do you think they got so ugly?
For players, when you go to try a horn always pack the following essential items:
For non-players, you should try and examine the horn as closely as possible. If it is a used horn, spend at least 10 minutes just examining the body for damage. If you have a saxophone-playing friend along, make them test every note against a tuner, and play loud and soft. Also, try a chromatic scale slowly from the lowest to the highest notes so you can check for leaking pads. An advanced "trick" is to play the overtones of Bb, B, C and D and compare them to their fingered counterparts. There will be timbral differences, but on a great horn, there will not be a change in the intonation. I also test close multiphonics to see how the acoustics of the horn react.
When it comes to being prepared to check out a saxophone, you can never have enough stuff with you to try them out. If you are going to look at a used alto saxophone, and have mouthpieces for soprano, bari and tenor, bring them along. I once went to check out an alto and found that the guy had several mint condition sopranos, tenors, and a few sopraninos. Had I brought my other mouthpieces, I might have been able to try them out. The moral of the story is to always be prepared. You never know what you may find when going to look at an instrument.
One final tip -- always bring a small container of rubbing alcohol or some other safe anti-bacterial substance if you don't have your own mouthpiece. I have seen some really gross mouthpieces, and if a gross mouthpiece is all that's holding you back from playing that mint condition, gold-plated, 1944 Selmer Super Balanced Action, chances are you'll risk playing on an unsanitary mouthpiece just to try the horn. Don't. Your health is more important, so bring along some anti-germ stuff. Remember to always practice safe sax! (I couldn't resist.)