A friend bought an Airstream and found the seller via Ebay. My friend made a deposit via Ebay's online banking service, (Paypal), and then made the remainder of the payment face to face, in cash, when he went to pick up the trailer.
This offered my friend reasonable security against getting ripped off, and the seller was fine with it, but Ebay wouldn't acknowledge that the payment had been made. They have no mechanism to register a payment made through any non-electronic means that isn't linked to their process. This was going to cause problems for both the buyer and seller, even though they both had what they wanted, because they both looked to Ebay, to have not gone through with the deal. They finally settled things with a service rep and 1/2 hour on the phone, but it was a hassle.
I'd be very leery of paying in advance for something that might not be there when I arrived, or might be in seriously different condition than what I expected. I'd be willing to send a small payment in advance, but I'd want to see and examine the machine first hand, and check the VIN and title, before I'd make the bulk of the payment. I would however, make the payment using an app on my phone, while I was live and in person, if that's what the seller wanted.
There are more scams these days than can be imagined. I've seen a bike for sale on Craigslist at less than half of market cost, and the same bike, same pictures, claiming to be at four different location around the state. Thank God, I wasn't taken in, mainly because the guy never returned my phone calls.