Most used laptop sellers will not give you a guarantee, because, they can't acertain if it will work fine for you. Once you purchase any used laptop, you are left on your own. If it develops a fault, sorry for you, and if it did not develop a fault, good for you.
I personally do not recommend used laptop for laptop buying. I guess when used laptop sellers buy this laptop, they bought it in a condition where it could have undergone many repairs, or developed a non-visible fault.
So when you take the used laptop home, after few weeks, you start seeing those faults that were not visible at first. The only good thing about used laptops are that they come very cheap, and that is because it is not new. They may also look good for its being cheap, but they are not guaranteed working machines.
My advise is this, if at all you are going to purchase a used laptop, go with a laptop expert, or even a laptop repairer, so that it can be properly examined before you pay for it.
Another advise is this, make an agreement with the seller, that you will probably return it if it develops a fault within a certain period of time. Take note, this is not a guarantee, but it may help you get some audience when you retun it in case of faulty performance.
Used laptops are the best, but they can provide you with a working machine at a low cost.