Ask around your friends, work or school colleagues, facebook etc. Where I work, several colleagues have sold their Macs when upgrading. It takes sometimes a really faint rumor to get several people interested. Plus when you know the person selling (or it is the friend of A friend, not a friend of a friend of a friend..) it is much better if there would be any problems when compared to an unknown ebay/craigslist ad.
And when buying even from a friend, make a written deal ("I so and so sell the mac with these and those specs to this and that person for that price on day xyz place zyx" - even handwritten is ok), just in case.
(Three of my Macs I've sold to colleagues, one (and two PCs) to a relative, one Mac via an ad, and I've got myself the Dell mini via a Craigslist ad, when mr. Giaguara had a work colleague in common with the seller who was working at Dell - so I always try to get the Macs a new loving home and cover myself and the buyer/seller as appropriate)