You want to sell your house and the prospective buyer wants to buy it. The purpose of the contact is to ensure that each of you gets what he/she expects to get.
In other words, this is the place to spell out with as much precision as possible the terms of the offer and in turn, the acceptance. A poorly worded offer or contract may simply be buying a lawsuit instead of a house.

Contract ABC'S guides For Sale By Owner sellers through:

The basic contract equation

The opening offer

The acceptance

Handling earnest money

Common contract contingencies

If you need a lawyer

and much, much more.