Best of luck on your sale. I will also be selling a 2011 iMac soon.
Based on my experience with selling Apple and other electronic items, here are my recommendations for a sales channel, from best to worst :
1) If you have a "NextDoor" forum where you live, this is the easiest and lowest cost way to sell. No charges for the ad. People will come to you for the sale. When you list the ad, you can select whether you want to restrict it to just your neighborhood or to a wider local area. I've sold several Apple items this way. This is especially good for an iMac, as you can avoid shipping costs and hassles. I restrict sales to cash or PayPal.
2) Craig's List. Many of the same advantages as above, but more hassle and exposure to fraud and questionable buyers.
3) The MarketPlace forum here at MacRumors if you qualify for it. You might want to start with a local sale ad first to avoid shipping.
4) Ebay. Fees, hassles, shipping and ever-changing rules.
5) Sell it back to Apple. Very easy and trouble-free, but you will probably get less $$ for it.
Of course, you can try multiple channels at the same time if you wish.
Whatever channel you choose, you can maximize your return by taking good photos of the iMac and accessories, including all packaging, manuals, etc. Also provide a record of you purchase date and costs. And make sure you are asking a reasonable price for a 7 year old computer. Check recent sales (not asking) prices on Ebay for guidance.