Definitely sell on eBay as "buy it now" NOT AUCTION. You have higher fees selling as auction and you make less. I sold my one year old 64gb 4s for $600 on eBay. I got $540 after all fees. I paid $399 for it a year ago(With 2 year contract). N I paid $9.99 on an eBay listing to get it factory unlocked before selling
I am also selling a iPhone 4 32GB Black Factory Unlocked on eBay, Craigslist, Kijiji and on here but I'm having a difficult time selling it. I am willing to negotiate a price but all I come across with are scammers or with people that end up changing their minds. I have been looking at trade-in programs at places like Gazelle.com, Apple's reuse and recycling program, Glyde etc., but they all seem to need a US address to complete a deal. I live outside of the US and in Canada.