When you are sure that you have totally finished with a particular record you may want to finalize it. By finalizing a record you are making it read only, note that once finalized the record cannot be un-finalized, so only finalize when you are sure you will not need to edit it again.
When you finalize the record Onyx creates an historical record of the sale along with a snap shot of the purchaser's, vendor's and vehicle details. If you do not finalize a record then any changes you make to the purchasers account will also be reflected in the record should you ever re-edit it.
Additionally, Onyx will not let you delete vehicles or accounts that are linked to non-finalized vehicle sales records.