the total must be at least 0.0001 is the "error"message who comes when I try to buy without having enought coins.
The minimum Ether trade size is indeed 0.0001.
What happens is that when you place a sell order of 0.1ETH for example, and another trader buys 0.09999999ETH, your order remains open as 0.00000001ETH because it's not completely filled. If someone buys into the orderbook after that, this 0.00000001ETH order will appear as single orders in the trade history list. It all comes down to how much balance people have to buy or sell with, and these balances mostly consist of odd numbers.