I often buy out of state.. current camper being one of them. In just the past 4-5 years, I've picked up a boat and a couple of 550's long distance too.
Normal course is locate what I want, get in contact and have a chat. Phone calls are amazing and without it, I wouldn't move forward. As in our camper, the seller had already posted good pictures. I asked for a few more. Whether they come through or not, how decent the shots are, timing, etc, all comes into play I guess. Kind of an informal vetting process.
Most of the time I'll offer up a deposit. It doesn't have to be ridiculous..500, 1k for a 20-30k item seems reasonable. You're telling the person you're serious and asking them to hold something. If you fail, they deserve to be compensated for taking the risk. This whole thing works both ways. Most of the time, this isn't Ebay with all the protections. You need to trust and they do too.
Hook a flight up and fly and look it over. I've had 40 min deals, I've been picked up at the airport, I've used Uber... hell, I slept in the camper we have now one night in the guy's yard because of the timing of the flights.
Cash is great... or you can just run to the bank together. Cashiers check..whatever you're both comfortable with. Vehicles, I usually just jump in and drive home. For the camper, I bought it, flew back home and hung it on U-ship until someone bit. That of course, was worked out ahead of time with the seller.
I suppose if you weren't comfortable leaving it 'there' until you could get it picked up, you could work a deal to have it dropped at a storage unit for a month, or something. Options are only limited by you.
I've not only found deals that were worthwhile and offered large savings, but in the case of trucks, I've been able to locate what we want...which would rarely happen just hanging around home.
'16 F550 CC, 4x4 with Link Ultraride air suspension, '18 AF 1150. Just so we can play with our snowmobiles, dirt bikes and fishing boat. And new 20' tag along...kayaks, bikes, mc's and extra water and food!!