you can return clothes bought at Biedronka (maybe at other places too, like Lidl, etc.) during 5 days due to the fact that you can't try them on in the store, only at home.
Exactly. The law says that they have to make conditions to try out the product in a store, so if they are not able to do it, they usually let try it out at home and let return it after a number of days.
I don't see why the store would have to take the mouse back, especially if it was opened and used. Stores are stores and not some rental places, after all.
Exactly. You can always ask the store staff to let you try the mouse out in the store. In big consumer electronics stores it's usually not a problem. In Media Markt and Saturn they have even recently started to put the remotes next to the TVs on the store exhibition, for customers to be able to try them out easily. Or when I was buying a vacuum cleaner, it wasn't any problem to connect each one in the shop to the power outlet and check how well it sucks (when most of the things suck, it's not good, but when a vacuum cleaner sucks, it's good, isn't it?).