Ok, so Gazelle buys used cellphones and such. Where do I buy one?

I read about a company called Gazelle – actually there was an interview with the CEO on an NPR show over the weekend. Apparently this company buys used cell phones, tablets, etc., for cash, then cleans them up and sells them (presumably at a bargain).

I went to their site – it is all about how to sell TO them. But what I don’t see is how to buy some of these used phones at a good price.

How do they get rid of them? On Craigslist? In bulk overseas?