There are two common reasons why folks are interested in purchasing a used computer:
1. Buying a used computer is like buying a used automobile. Sometimes folks cannot afford to pay $30,000 dollars for a new car...so they buy a 3-4 year-old "used" car for $18,000.
2. Just like a new car...new computers depreciate VERY quickly. Why pay $999 for a new MacBook...when you can buy a 3 year-old MacBook for $350 (which I have done).
How many of us on Mac-Forums have purchase a used car...and are currently driving a used car? Would we rather be driving a nice brand new car...SURE!!!
...buy sometimes it doesn't make financial sense to do so. An automobile is just as complex as a computer (lots of parts to go bad, lots of electronics, not to speak of traffic accidents)...and an automobile is just a mobile as a computer...so is just as likely to get "broken" or damaged.
And...I would suggest that the average member on Mac-Forums is no more knowledgeable as far as being an auto-mechanic...as they are knowledgeable as a computer repair technician. So there is probably just as much risk for the average Mac-Forums member to buy a "lemon" used car...as a "lemon" used computer.
Of course the very very important common element here is the "test drive". If you buy a used car...you test drive it. Same thing applies with a used computer. It is best to take it for a "test drive" before buying. This is where Craig's List is a great place to purchase a used computer (test drive before buying). e-Bay is a bit more complex...but there is a recourse system when buying on e-Bay & when paying with Paypal.
So again I say...if someone is ok with purchasing a used automobile...purchasing a used computer is not a whole lot different.
- Nick