I bought a D300 nearly four years ago, when they first came out, and kept my D50. I still use it a fair amount, even though I have another camera. One advantage of the D50 is that as a 6mp camera its file sizes are smaller. For most things I do I don't need more than 6mp (and you can produce very good-looking large prints with only 6mp).
I would say it depends on whether you would use your D50 as a back-up or second camera. If so, it seems worth keeping. But if you wouldn't use it, then it makes sense to sell it to a friend rather than leave it unused. Marty has offered some price guidance, but if it's really a friend I wouldn't think the price would be a really big issue. (Of course, both of you want it to be a fair price.)
If it were me, I know I'd keep the camera as a second camera, because that's what I actually did, and I actually am still using it.