Actually I would do all of the above; MacRumors marketplace, CL, Reddit, Let Go, and that facebook yard sales website.
With an iMac that is easily damaged when shipped, I would try to stick to local.
Just be carefull how you setup your accounts, consider making a gmail email just for selling. I only left out ebay because they get a piece of it and its binding once you put it up for bid.
I will list things on all those sites at the same time and old CL seems to sell the item almost every time. I always assume the worse from CL, but did get a good deal on an iMac for min and my parents iMac's.