I'd really not recommend buying a used laptop for gaming 'usually' used gaming laptops are being sold because they have been abused , I.E. run on the max with no additional cooling pad / protection , the insides re usually fried I know of one friend who has to get his alien-ware's graphics card replaced on an 6-8 monthly basis ( this is the soldiered on GPU) so he's very fortunate the people who run his store extend his warranty
If you are going to game on a laptop you will need to take extra cooling into account for sure some kind of cooling station will be mandatory because the laptops are not designed to dissipate the heat their parts put out for > 30 minutes or so and that's hardly ideal for gaming.
Anyway back on topic it...