A survey description should adequately indicate where the sale is to occur. So have one written up if possible and include it in your timber contract. You will also define the trees to be cut or left but do this as is suggested in a later step.
A good sale description significantly improves the contract and will protect the seller from any misunderstandings. Describe the area as accurately as possible. This might, additionally, be supported with flagging or paint around the sale area.
You also have an obligation to guarantee good title which gives you the right to sell. You will be asked to assure the buyer of this fact in a later step. Double check the description and deed of the property where the sale is to be made to insure that you have the right to sell all timber in the defined area and who else (if anybody) has an interest in the timber. This is very important when the time comes for signatures.
See Check Step Four - Sample Timber Sale Contract Template

