I want this information to be available for everyone so posting it here. Someone is trying to sell used BWT filer on HB and I remembered this from BWT manual. It applies to filters from other manufacturers as well.

5.3 Procedure after breaks in operation
In the event of prolonged breaks in operation, close the check valve in the filter system intake. After a break in operation exceeding two days (weekend, holiday, etc.), the filter system
must be flushed with 4-5 litres of water before being reused.
After a shutdown of 4 weeks or more, the filter cartridge must be replaced by a new one.

Since this is impossible to verify, I personally would never buy used filter....