Hi Les. Difficult to give exact advice but this will start you off and others will advise further.
First off what is the asking price for the laptop, new basic ones can be bought for a very reasonable price now. What is the condition of the outer case like, does it show signs of a hard life?
The screen, is it in good condition? Check the brightness/contrast has not been turned up to Max just to impress.
Apart from the screen itself, the next most expensive part of a laptop can be the battery. Is this in good condition? try to have the laptop running off the battery only for about 1 hour while your chatting. Take a DVD movie with you and leave it running.
Make sure you have the disk for any programs/applications on the laptop. if not then reduce the asking price. Open/close several applications at the same time to see if it can handle it. Take into account the age/condition and price. good luck. j.