Don't buy or sell a used or new computer with out a bill of sale that has the details written in it for future reference.
You might have to refer to it in the future and you will need proper paperwork to back up the sale.
Whether it is for a desktop computer, a laptop, mini-pad or tablet be sure to get the back up paper work so that in the event of any incident you can prove the purchase was legitimate.
This way you have the details of the agreement on paper.
CAUTION:
Don't write a check, use a credit card or even make a cash transaction with out the Bill of Sale to finalize the deal.
If you are selling it your customer will feel much better about the deal if he/she has a
receipt for the computer.
The buyer paid, and will feel better
when he/she knows that a bill of sale is proof
against any claim or to prove that it is
the same item that was sold.
As a seller you will be doing the right thing and if you are buying it you will feel that you have been treated fairly with a Bill Of Sale.
You most likely will want to do business with the person in the future.