Experience knowledge of what the legitimate ones look like subtle differences in calligraphy surface appearance other details such as denticals, rims, etc.
The two experts who have spoken said they are suspicious, not certain. You have to understand that it is often possible to be 100% sure a coin is not genuine based on pictures, or even a description; but it is almost never possible to be absolutely certain that it is genuine without an in-hand examination. For example, if the weight or diameter is off, or the legend is incorrect, you can be sure an item is counterfeit; but correct weight, size and legend are never enough to authenticate. I am uneasy about your coin, but can't put my finger on anything that clearly marks it as a fake, so I will join the others in not absolutely declaring it fake.
Should I take it off eBay? I did say on the auction that I wasn't sure if it was fake or not. What should I do?
Whatever your conscience dictates. If it were mine, and if I were an EBay seller, I would have authenticated it to my own satisfaction - probably with the aid of an expert's examination - before listing it.
hontonai is really an impressive coin collector and i love his attitude and especially his description about the points of his reasons of judgement!