I have sold old photo equipment to fund my purchase of something newer and better. For me photography is a hobby, not a profession and my budget is too limited not to consider selling old gear.
I sold a Pentax Me Super + 2 lenses to buy a Nikon N75 + 2 lenses.
I sold the N75 + lens + flash to buy a D50 + lens.
Today my D50 + 4 non-VR lenses would be considered obselete by most people who read and post on this site. Yet this very basic consumer DSLR has never let me down, never needed repair, and still works very well. I still like it better than its successors (D40/D40X/D60/D3000). More important, it has been the vehicle for me to learn a lot and become a much better photographer than I was before owning it. With it I shot some photos that I am very proud of and with it I shot quite a few really lousy photos, but they were not that fault of the D50. My skill and creativity, not my gear, has always been my biggest limitation. Not being able to afford the latest and greatest forced me to concentrate on improving my photographic skills and that was a good thing.
But now I have outgrown the D50, had thought of getting the D90, but the D7000 at a price of $1200 (a stretch for me) offers a feature set I thought I'd never be able to afford short of winning the lottery.
Numerous times I have wished I had two bodies, with a different lens on each. I'm probably being sentimental, but I'm going to keep my D50, even after I get the D7000.