While Internet research is important, you should also do some "hands on" research. Go to the stores, lift the laptop, feel the keyboard, open and close the lid examine the screen resolution. There is no substitute for feeling that the fit is right for you.
a. It is likely an "as is" sale. Some issues with electronics are not easy to spot.These are substantial risks so I look for the savings difference to be commensurate with these risks. When calculating the savings, do your research to make sure you are paying today's prices. A computer may have sold for $600 a year ago when it was new, but similar new computers may be selling for $500 this year. If you buy it for $450, you didn't save $150, you only saved $50 and was that worth foregoing a warranty? Maybe not.
b. Cigarette or other smoking sellers. Unfortunately, computers draw air through the components using fans and offensive smoke can and will be drawn into your computer. This is something you may not think about until you get your bargain home and then end up dreading turning it on due to it spewing second hand air your way.
c. Is it stolen? Look for indicators - Is the seller too eager? Is the deal too good to be true? What is the motive for the seller to sell?
d. Ask for Restore Disks: I recommend this for two reasons. First, if the individual took the time to burn restore disks, it may suggest that they cared about the computer and took good care of it. Second, having restore disks will make your life much easier - there is no need to scramble for a legal copy of an operating system and it is a nice way to get rid of potential files (and even malware) that may be on the machine.
Your laptop quest should be fun, but be careful not to get caught up in the excitement and be prepared to walk away from a deal if it doesn't feel right to you.