When the buyer and seller have to consider and agree the agreement. The seller will be the key document is the contract of sale (Sales Confirmation, Contract Sales) or a Proforma Invoice.
When the buyer and seller are considered and agreed to the deal then. The seller will not have to do the important agreement (Sales Confirmation, Sales Contract) or a Proforma Invoice.
When buyers and sellers to consider and deal agreed. The Seller shall have the important document is the contract trading (Sales Confirmation Sales Contract), or leaf Proforma Invoice.