I would only buy a used airbrush from a cake decorator. You have no way of knowing what else went through the brush. Some paint materials are toxic.
You can get a very inexpensive airbrush from a craft store for less than $30 (even less if you have a coupon). I only have airbrushed dummies, so I use the propellant in the can. I don't want to buy a compressor just now.
I think if you're to the point that you want an airbrush, invest into a good one because when you've got a not-so-good one, you'll have more problems than what it's worth. Mine is an Airmaster (Kopykake) and through ICES and ACD they often offer a discounted rate part of the year. I'd rather go new because then you know exactly what has been through it for safety purposes. Otherwise, check ebay...
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