The EU 14-day online returns is an extension of the older regulation against high-pressure sales tactics, like telephone sales, door-to-door sales, and booths on the street.

It allows you to change your mind if you made an impulse purchase, but not really to "test" a product. The returned item must be unused and in new condition, with any seals unbroken - generally if you open the box or rip any plastic bags the online retailer doesn't have to accept a return any more.