I've done a lot of shopping for tractors, cars, trucks, motorcycles, etc over the Internet, and here are my own rules of thumb. Your mileage may vary...
1. I try to buy from original owner, so I can learn as much as possible about how the thing was used and service. The more service records, the better. (With certain cars, for example, I won't touch it without complete service records.)
2. For something like a tractor, I try to find out if it was dealer serviced, and if so, try to find out where and see if owner has any complaints if I contact the dealer and see if they know the machine.
3. If it's not being sold by a dealer, I sometimes ask the seller if they'd mind getting it over to a dealer for a pre-purchase inspection, which I pay for. That's $150 or $200 well-spent. That's a little awkward if it's being sold by a dealer though.
4. If it is being sold by a dealer, I found out where the machine came from: was it a customer trade-in? The ideal scenario, in my mind, is a one-customer trade-in where they've had a long-term relationship and have done at least some of the service. Sometimes dealers will offer me the previous customer's name and number, so I can talk to them directly. That's always good.
5. Obviously pictures are necessary - the more, the bigger, the more clear, the better. Many indicators of overall condition can be seen in pictures, but they don't tell the whole story and they're always better than the thing is in real life.
6. If something doesn't sound right, if you have any reservations, walk away. I've ignored that little voice that says something isn't adding up before, and regretted it.
7. Learn all you can about the machine (by searching TBN, for example), and identify all the weaknesses and potential trouble spots for that particular model and set of equipment. Compile a list of questions that cover these areas, and also questions about general service history and oparation. As you go through those questions with the seller, you'll quickly get a sense for how committed to maintenance they were, how much care they took, and how well they understood the machine. My favorite type of seller? The perfectionist who obsesses about maintenance and apologizes if there is a small crack in the seat or something.
8. Sorry to be wordy, but my final tactic is this: if you cannot go there to see it in person, and you do not have an arrangement with the seller to back out if you don't like what you see, then don't risk more on a deal than you're prepared to lose. I bought a motorcycle for $2500 sight-unseen, and had it shipped. That's the only time I've ever done that. It worked out, fortunately, but I put a lot of scrutiny into that deal and it was actually better than I anticipated.