I have a late 2009 27" iMac with 16gb ram, i5 processor.... The graphics card started to fail (or so apple told me). Rather then put money into an older machine, I bought a new one today. My question is, is an older iMac worth anything? If so where would I sell it? Another thought I had was maybe I could use it as a second monitor, but then I remembered the failing graphics card. I guess I could repair it if it could be used as a second monitor or as a primary with a Mac Pro. Thoughts?
Sure, it's still worth something. Not much, but something. A 24" 2008 iMac with a failed GPU sold on ebay for $142. So yours is probably worth something in that neighborhood: $150-$200. Best place to sell used computers is on eBay or craigslist.
I would recommend the Marketplace forum here, but you need a certain number of posts before you can put something up there for sale. I want to say it's 50 posts.
I was able to sell my old 2006 MBP on Craigslist pretty quickly (inside a week as I recall). The screen was shot and it didn't even boot up, but the guy was just interested in the parts and I got $160 for it. Chances are good that if you live in a sizable metro area there is someone (hobbyist or otherwise) that is interested in your old machine.