Back in the day, I collected their promo displays, when I could get them free from record shops. I had the complete display for "The Visitors", but it must have got discarded at some point as it was too big to carry through life.
I did, however, keep the UK promo posters for the singles "I Do I Do I Do I Do", "Mamma Mia" and "Fernando". These were interesting in that they were issued before ABBA's major success in the UK and are therefore very rare - I have never seen them elsewhere, even on the Internet. Because they are so rare, I felt they should be preserved for all time and I was worried about someone throwing them away when I've gone. So, last year I donated them to the ABBA Museum. I will post a photo of them when I can get the image upload to work.