Or just keep a book and quill with you, keep down the history of your items, say, you had a silk touch I, efficiency III, You write it down in the book. Once someone buys it, go to that item, write down the amount of money they payed you, and then just, let's say they payed a portion, just write down the amount they payed, and write down, owes 500 dollars more, and he payed 4000.