I have never found used laptops to be a good deal. If you browse the relevant areas of fatwallet.com or techbargains.com, you will often see new laptops in the $450 +/- $50 range that are far better than similarly priced used laptops. You won't find one that's 2.8 lbs in that price range, unfortunately - or if you do, PM me so I can buy one!
Other than making sure it has built-in wireless networking, my advice would be to check those sites for deals and then decide how much $ it's worth to you for a lighter laptop. Most low-end laptops are in the 6 to 7 pound range, not including a case.
Also, if you buy used, count on buying a new battery pack in the near future. Those puppies are usually in the $100 range.