Hi C.,
I'm a flute teacher, and my advice is to find someone who either teaches the alto sax or who plays the alto sax well and who can evaluate any instrument that you are thinking of buying. It's easy to buy something online or without playing it and then find out that it costs more money to fix the instrument than it would have to rent/buy a new/used one. Repairs can be very expensive, and you could end-up with an instrument that isn't worth anything (due to the needed repairs) and isn't playable. To use the flute as an example, an overhaul/repadding can cost $100-$300, and many used instruments won't play at all until this task has been completed.
Good luck! I bought flutes online for my children to play, but I'm a professional flutist and I knew what I was looking for and I made sure that I had the right wording in the contracts so that I could return the instruments if they did not play well enough. I also bought brands that held their value and that would have good resale value once they were reconditioned/repadded.
A. C.