I've been looking for a D25 (first early flat back and now mid 70s arch back). Obviously, you cannot find them at every local GC. I cannot tell you how many Reverb sellers have a no return policy and also don't list detailed information about the action or bridge/saddle. With a neck reset potentially adding 50%-70% to the cost, I am limiting myself to guitars with at least a 1 day return policy. Some ads say "low action", but when asked either don't respond or will come back with 7/64" or 8/64" (not what I'd call "low"). I'm one of those sellers that lists neck widths and thickness at the 1st, 5th, and 12th fret and the action of the e strings at the 12th. And I give a 3 day return window. But I feel like I'm an outlier.
Anyway. So what do you look for? What risk are you willing to take? Am I being unreasonable is messaging and asking sellers about this?
I am not saying these apply to the people here but for the majority of others on the internet...
If the offer is too good to be true, it probably is.
The guitar is almost never as good as the seller claims.
The pictures posted will always show the instrument in it's best light. Not saying the pictures are doctored, but for listings with 3-4 pictures, it's guaranteed that they are glossing over something.
For instance, someone posts a Guild F512 from 2020 for 500 dollars claiming it's "mint or excellent condition" but only has 2-3 photos with low resolution, it would be red-flags galore to me.
Next. Be sure of what you want and what you will accept. Are you willing to accept a top that developed a split and has been fixed well with glue and a cleat or cleats as necessary? What about if they fixed it but they left a lot of glue? What about a bad repair job? Note: Almost no one is going to say it's had a bad repair job or that they made a hash of it when trying to fix it.
If you are buying an antique, how much variation from original are you willing to accept? Replacement tuners? A refinish? One that was destroyed and then had a complete rebuild, including a new neck?
If you are buying something "owned by someone famous" or some other notable example of a guitar, what kind of providence are you willing to accept to "prove" that it was really owned by that person? Example, someone claims to own one of John Denver's 12 string guitars. Yes, you can find out if he owned that model, but what do you need to be sure that he actually owned (and maybe even played on stage) that particular guitar? If it has a signature, has that signature been verified by an independent third party that can be trusted? Do you have the ability to independently verify the signature/providence and return if it's not as advertised?
Ideally, I would also ask for a sound sample, but not everyone will be able to either record it, or be able to even play. Some of these instruments might just be owned by a collector that saw them as an investment and they don't know how to play. They might also be the heirs of an estate that has looked up "grandpa's guitar" and see's money signs flashing.
Finally, remember that the more expensive an item costs, the more likely it is that someone will try and counterfeit it. Again, not talking about people here, but I go back to the first warning...if it looks too good to be true, it is.