I'd NEVER buy a used laptop.
As for "gaming" laptops you need to spend quite a bit to play the good titles. If you have $1500 it's better IMO to buy a $400 laptop and spend $1100 on a desktop.
If you have say $800 to $1000 then I'd probably look around for a laptop based on NVidia's 600M Optimus series.
If you have $500 to $700 it might be better to buy an AMD based system with HD7000 or HD6000 graphics (HD7000 much cooler/quieter but may be some HD6000 deals).
*When it comes to gaming on laptops, carefully choose games that work well on your system. Some games scale well, some don't look that great unless you have a MINIMUM spec'd machine.
Diablo 3/Torchwood are examples of games that are well coded. Torchwood 2 would be my top recommend for a laptop game (with a mouse). On the other hand, Batman Arkham City might not work great.
*The single largest mistake I see in PC gaming is people increasing the graphics quality too high at the expense of framer rate and stuttering.