Any Mac reseller would buy your iMac from you - but the question is, how much do you want for it? Be advised that resellers will buy your Mac for significantly less than what it's worth (and less than what a private buyer would give you for it), so keep that little fact in mind.
Try to sell it locally, or on ehMac, for example. What kind of iMac is it? Specifications?
If stock, and in good condition - no less that $400 privately - although a reseller would offer you $200 or less - at least, that's what I'd predict. I've gotten quotes from resellers on my upgraded iMac DV 400, and it was pretty close to that amount. Pretty lame, if you ask me.
why dont you just put an ad on ehMac, and the buyer has to pick up in person with money order or cash or bank transfer or paypal if you want
You will get the most if you sell direct to a customer
and ehMac really does work well for selling off your used Mac