I think the going price for that configuration is around $1800, right? If so, then since you used it for 10 months and it's still eligible for AC, then I think I would look at the refurbished price and then adjust from there. The current iMac refurb model available is the 21.5" i5 with 4GB RAM and 1TB HDD and going for $1249($1365 including taxes) and still eligible for ML up-to-date program and will come with the initial 1 year of AC. Given the slightly faster processor and more RAM, I would guess that $1500 might be a reasonable price to ask.
The reason I say that is because a 16GB RAM kit only costs $110 which would still actually keep them under the $1500 mark and still have coverage for 3 years of ownership while buying yours only gives them 2 years and 2 months.
I dont know what your reason is for wanting to sell it, but if it's so you can upgrade when the newest one comes out, you might be better off just upgrading it and making it super awesome. Some sites say that your iMac can support 32GB of RAM with 4 8GB modules. That much RAM and a SSD would be great to see.
EDIT: Just realized I hijacked the thread and was way off topic... use the refurbished price as a GUIDE but DON'T assume that everyone knows about the refurb apple store, because they don't.
What you have is the newest model (until they come out with the next one) so now is the BEST time to sell. Keep in mind you're already eliminating the tax burden on the purchaser as well as shipping & handling.
I'd ask for $200 - $250 less than you paid for it, not including the ram upgrade. I'd sell it with the specs it came with (4gb) because then you can advertise it as a high-end 21.5"
Stick to your guns on what you want for it, don't take low ball offers. You'll find someone willing to pay what it's worth. There isn't a more successful company on the planet than Apple, so it isn't a hard sell!
Off topic Craigslist info (original post):
I think you can do a lot better on craigslist than you can on ebay. It just takes some more time. For whatever reason, I don't start getting serious offers from legitimate people (who speak in complete sentences) until about two months after listing.
In my posts, I always put that we will meet in front of a sheriff's or police station, no exceptions. Also, people are just as wary of 'too-good-to-be-true' prices as you are, and they also equate a higher price to a higher value (unless you're being ridiculous.) If they are still interested and contact me after that point, I figure they're not looking to rob me, or to offer me $300 less than I asked.
Anyone who makes it to this point, I just end up meeting at my house (if they're comfortable with it.) I've met more professionals and made more high end sales through craigslist than any other way. Safety is always an issue though, so do what you're comfortable with.
If you're anything like me, you're a professional who has a very well maintained computer, looking to sell it to someone who will appreciate the care it's been given. Last month, I sold my 2011 21.5" iMac (base model) for $1050 cash. Guy came to my house and met me, he was a local musician; now we play diablo 3 together.