Agreed. But when a BUSINESS does a transaction, cash is not preferable. A $6500 cash transaction borders on money laundering. The BUSINESS also needs to pay sales tax on any purchases, whether it is a vehicle, a couch or a lawn mower - even if purchased from a private individual.
The government loses the ability to track income and expenses if you use cash.
There is categorically zero wrong legally with paying in cash. It is not illegal to deposit cash. You can even deposit $9,000 at a time a few times in a row and it's not illegal structuring ($10k being the limit where you must file Treasury paperwork). I've deposited a series of $8-9k cash deposits, I mentioned to the teller what I was doing and said I wanted to make sure they and I are in compliance in case they need to file a SAR. They said it was fine at the level I was working at.
You just can't split a transaction into multiple parts to get below $10k to avoid the reporting requirements - that's a violation of federal law (learned that in law school!).
This was a one off transaction, below the $10k threshold so zero risk of it being illegal in any way, shape, or form. Maybe the buyer gets called by his bank, and I assume he tells them the truth - bought a van on craigslist, here's the buyer's name and # if you want to verify, here's a photocopy of the transferred title.
As for tax compliance, when you register the car and have a new license issued, our state assesses a 3% use tax (= sales tax basically) based on NADA/KBB value, not actual sales price. So the guy is going to pay 3% of $6000-7000 or whatever the tax office carries this make/model/year at. No way to avoid it, though maybe you can protest that it's an old, crappy vehicle, broken down, worth less, etc. But I don't recall that being an option.
As for the government losing the ability to track income and expenses with cash payments - that may be true, but there is a very clear paper trail that a college intern at the IRS could track (again, assuming he actually withdrew the $ from BofA which the envelope said when he handed it to me).