In the last 2 months I listed 3 DAPS in the Head-Fi Classifieds, two of which were sold to people from the forum. The third was eventually sold on eBay. As
JoelWilcox said, PayPal account (for invoicing and secure payment) is probably a must. I also purchased a DAP off a private seller via bank transfer, but I would not recommend this, unless you have strong evidence buyer is trustworthy. In my case, seller went to a whole lot of trouble to provide evidence of trustworthiness and it was a very fortunate transaction. Reddit also have a dedicated selling forum for audio equipment, but again I'd be cautious.
Edit: I also tried Facebook marketplace, and got a lot of phishing, obvious scamming approaches. But I listed one item on Gumtree and got an honest and interested buyer (who got beat to the line by somebody else on eBay). Listing on eBay, also, entitles people to PayPal credit (whereas private forums or Gumtree do not), you should keep this in mind as well.