How to ..
I have a service which is a monthly Second Hand Mail Out where anyone selling pre-loved Macs and related things can list them for free. Subscribe to the monthly Newsletter to receive them both into your Inbox.
When buying a second hand Mac care should be taken as with buying any old equipment. Computer years are like dog years! Anything older than 4 years is ancient — although it might be in perfect working order it's days are numbered by the seller probably paid a lot for it and wants to get some money back. Look on the Apple Store to see the prices of similar new machines to help you make a decision on what it's worth.
You can also check these sites to look and see if there are any similar machines available giving you an idea of market prices. Microseconds and AppTech have a stock of second hand macs and peripherals so check them out first.
If any warranty is available obviously that helps even if it's nearly run out. Then general condition when you see the machine is important and what operating system it is running, how much RAM it has and what capacity hard drive it has are all factors. To get this info visit the page Finding Information About Your Mac.
Laptops usually have a harder life than desktops and this is reflected in the price Apple charge for extended warranty: it costs almost double! The condiiotn of the battery is important as well as a new battery is around $160 and they have a life of about 3 years before replacement. All things to consider.