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 all depends on where your based but I'd say your best bet was actually a dell (yes i know shock horror)
but at the price level they really are hard to beat and some of their inspiron 17R/SE models are very well specified out
i7-3630QM to 3.4 GHz, 6 MB cache
Windows 8 64bit,
17.3 " Full HD-WLED-screen (1,920 x 1,080)
8 GB 2 DDR3 SDRAM at 1600 MHz
1TB Serial ATA hard drive (5400 rev / min)
8x DVD + / - RW Optical Drive
2 GB nVidia GeForce GT 650M 130W
799 euro ( from dell.de) the 99 extra euro gaurentee's you free from buying used junk.
there are other options including i5's 15' inch screens etc ranging from 599 upwards