Get receipts for your face to face transactions. Problem solved. I really think you are making this out to be much more difficult than it needs to be.
Receipts are proof of nothing though, i could fabricate them all day, are simple unprovable records all auditors are looking for, or do they require proof, and if so what proof could you possibly give them that you sold your tv for $200 and not $800, or that you sold your tv and not an ounce of weed?