If it looks too good to be true, it probably is so take care as the price might be masking larger damage to the used car parts, or it could be vehicle salvage (HPI checker provides this information).
Finally test drive it and analyze the the car and the used car parts. Check the car engine sounds good, the car parts are in good condition, it drives well and does not make funny noises. I took one of the guys from work since they know what to listen for and its a good tip to ensure the vehicle is in good working order.
Finally use your nugget (head) for common sense issues, if the seller is pushy it probably has something wrong with it, if
they wont let you test drive the car ask why not. Another tip is always make sure there is at least 6 months MOT left and some tax since this is one more thing to show you the vehicle is in good working order.