There is no website that allows you to sell your laptop and then upgrade with the ones they have in stock. Some might agree to let you buy one from them, but most just exchange old laptops for cash. Details are available on getcashforlaptop.com/.
You can always sell a laptop on eBay or Craiglist, but it won't give you an upgraded one because you sold your old one. The might you a small reward for selling your old one, if you sell it to them and buy a new one from them.