There should be an option so that only the owner can break his own trading station

This was the case in old Adorn, but it was really buggy and led to unbreakable trading stations. Other can't keep "all the items" anyway if you have the locked trading station game rule enabled since only the owner can open it.

You can use a world protection mod to prevent breaking blocks if necessary, but that's not a decoration mod's job.

You can use a world protection mod to prevent breaking blocks if necessary, but that's not a decoration mod's job...
just use a chest, has the same use and has the added bonus of being able to accept payment with things you don't already own :)