Buying sight unseen or based on limited photos is always a problem. Lucky for me, I have never had to do that since I have been able to find tools locally. I would try to ask as many questions as possible of the seller, not only to try and determine the quality of the item but to judge the knowledge of the seller. If they really don’t much about tools then how can they judge the tool’s quality? I have had tools described in glowing terms only to find they had missing parts and major flaws. I would also ask about their return policy and look at their seller rating if that is available.
I know you have your heart set on a Disston but if all you are looking for is a user then don’t discount a new big box store saw, assuming you are looking for a carpenter’s saw. Most crosscuts are really rough cuts that get refined with further tools. I know there is a lot of pleasure in using a fine old tool but they usually require some TLC prior to being put to work and that requires a few extra tools and the skillset. New saws are generally ready to go and will do the work. I prefer the old saws but I have been at it a while and truthfully, if all I am after is to get though the board the new saws will get you there.