I actually prefer to buy from sellers that can't seem to work a tape measure, can't take a good picture, can't read the model name off the frame, and are really slow to return emails. Why? Because those are often the best deals, sometimes incredible deals. One great deal I got was on a nice road bike, where the seller couldn't figure out how to put air in the tires (he thought they were defective, the bike had Presta valves).

A really sophisticated seller will likely know their values well too, and the price is almost always higher.

The bicycle market is very inefficient, the person with the most knowledge usually wins. I would rather be that person.