Best place to Buy a 2nd hand Laptop is the Members Market on here but you are a few (83) posts short of the requirements, The MM is the Safest place on the Web to buy from though if you check Trust & pick the right Member/Seller. You could look on Gumtree & Swapz & even Ebay but only purchase from sellers who you can go to there location/home. You can fully test it, Check there I.D., Receipts etc & pay cash minimising the risk of getting ripped off.
I buy a few 2nd hand Laptops for myself, friends & family & have to say the deals you can get for your money are outstanding. Spec wise, Age, Condition is what's important & make sure you do plenty of Googling on possible upgrades before you make the final decision. Some Laptops are upgraded by simpling removing 1 or more screws where as others are nigh on impossible so do your homework.