I recommend getting in person help in choosing the instrument. Someone to play the instrument, run it through its paces. Someone to hear you play it. Can your teacher help?
With someone to help judge the quality you can shop anywhere.
I would not shop online. Shipping is a mess, and you need to be able to return the item if you dont like it.
Craigslist and facebook can be fine. But you need to bring an instrument to compare it to.
Most important in all circumstances is getting someone to judge the instrument.
Buying from a shop is usually good. They are in the business of selling good instruments. Ones that are set up properly, etc. Shops also have many instruments to compare. This is an important part of the process. Most shops will allow you to trade in an instrument that you bought there. They will not be as cheap as finding a great used one, or the pick of the litter. However, it is not necessarily easy to find great used ones cheap.
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