Many years ago at an early morning market in Fukuyama, I was looking at some Edo period coins. A middle-aged man handed me a bag. "These are all fakes. Study them carefully", he said, walking off. I was left speechless!
In my experience there are more real cash coins around. I have a box of Ichi monsen, yon monsen, ju monsen and Tempo hyaku monsen in various conditions. 一文銭、四文銭、(宝永通宝1708~)十文銭、天保の百文銭
As to Koban, they are so valuable it would be strange if there were not fakers out there. One coin dealer in Tsuyama looked at my little Hime-koban for me and told me that the better fakes are actually worth quite a bit; some people even collect them. There is an organization in Tokyo who will for a sum give you a certificate if they deem your koban to be genuine.
Hard enough to judge in the hand, the problem will surely be magnified buying off FleaBay. If you are up for a gamble, though...