Can I trade a Nikon D3000 for a D5000?

Like trade the D3000 then add $200 or more and get the D5000?

Added (1). I got it refurbished for $372

Added (2). Can I at least, sell it & buy the D5000?

In that case no, no store will take back a re-furbished camera. Trade would be out. Besides if you did an on line trade you might get $50 for your 3000, I know I looked into it once with one of my Canon cameras.

You'd might as well keep the 3000 and add a new one to your bag.

If you bought the Nikon D3000 at $500, depending on the store return policy for cameras, which is usually 30 days, yeah. But if you are going to add $200, you should get the D3100, which is a better camera

edit:
you can sell back a refurbished camera, yes, but it is going to be cheap. Better would be if you found an interested buyer and sell the camera and lens for him/her for $350-400, because that is all it is really worth now. If you manage to get $350 for the camera and lens, you would need to put $300 for the D5000

Yes, you can sell it and then buy the D5000, assuming you can find someone to buy it. Start by posting your ad on your local craigslist to see if anyone from around your area wants to buy it, then use eBay to sell internationally if you can't find anyone interested.

Unless video is important, you don't need the D5000. Lenses are much more important.