Which seat may be sold?
ECE
When buying a child safety seat it is crucial to make sure that it is supplied with an ECE label that confirms European certification. The ECE 44 is a minimum standard, which a child safety seat must comply with, before it may be sold. The ECE label on the seat proves that it has been tested thoroughly and that it complies with the current European safety requirements. The (orange) label is mostly in the form of a sticker or sewn-on label on the rear of the respective model and contains the following information:

CAUTION:
From April 2008, legislation (according to the StVO [road traffic regulations] amendment of 11 May 2006) that only child safety seats with ECE version 03 or 04 may be sold and used, applies. Seat models complying with older ECE versions (e.g. 01 or 02) must be replaced with the latest child safety seats.
Why is it important to use a child safety seat that complies with current test standards?
Used child safety seats, which are handed down from generation to generation, often seem to be in good nick. However, material fatigue or crushed padding is not always detectable at first sight. Even experts often have difficulty in detecting hairline cracks and to judge immediately whether an older model had been involved in an accident.
