1) the first thing is the warranty length of the refurbished computer. Many places on E-bay, etc. tend to offer only 90 day warranties which is not enough. Costco offers the full manufacturer warranty on its refurbished products (it has its own division that does the work) so you have the same warranty as a new unit and then you can just compare whether the cost savings are enough to buy used.
2) It is very risky to buy on E-bay unless you know how to research the seller. Make sure when you read the past buyer reviews that they have an extended positive track record of selling the product type you are buying as one recent incident I read they were selling something else and then suddenly switched to selling laptops and people were relying on the positive reviews for the other products. I am not an e-bay expert but I have heard this much about the process.
3) Unless you have confidence an e-bay buyer is legitimate and the offer (including warranty) is solid you would be better off looking at manufacturer refurbished areas (HP/Compaq, Dell, etc) as well as very reputable resellers like Costco. In addition, Costco offers a 6 month return policy on its computers (new or used) in addition to matching the manufacturers standard 1 year warranty. You can't beat that extra protection anywhere (in the event of a lemon or something about the machine that just doesn't suit your needs).
Is buying factory refurbished laptops on Ebay reliable if you go with a good seller? Or is it worth it to spend $100-200 more buying elsewhere?