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.
I actually saw this happen on other exchanges as well, and always wondered myself why some trades are allowed to happen under the minimum trade requirement, and your explanation has made the most sense so far. Probably is what's actually going on, as well.
So the 1 sat trades are really merely 'leftovers' from an initially bigger trade order, that later got filled through another order.
It's definitely weird to see so many 1 sat trades happen at once, though. Seems too much of a coincidence, so I dunno what's going on in OP's pic.
But ultimately, it's not going to affect the functionality of poloniex or any exchange, so it's not that big of a worry.