The best proof of legal ownership is the title. You may need to produce these documents to verify your ownership, especially if there are back taxes or tax liens owed. If for some reason these documents are misplaced or lost, an alternative document of proof must be presented such as a bill of sale. A bill of sale is a document written by the seller of a property to the buyer, transferring the ownership. It also acts as a sales receipt and documents the basic agreements of the sale. It works as alternative evidence that a property is now yours.