Good written records go a loooong way with anyone questioning your records. Although it is not as good as a receipt from the person, I believe that they are the ones that need to prove that your book keeping is innaccurate. If you walk into an audit with stacks of paperwork (purchases, sales, mileage logs, etc.), that show you are keeping good records, that should bode well with whomever you are facing.

This is just opinion and not based on actual experiance, so take it for what it is worth. If the numbers do not seem unreasonable they probably won't be questioned.