I do the same as Tiermann: I only keep some big boxes to store Playmobil in, and I have kept some old boxes that have the pictures of diorama's on the back, just for myself really.
I try not to think about sale value too much, as I'm pretty sure I won't want to sell my PM in the years to come. At any rate, used Playmobil sells well without the box too.
My collection is taking up enough space as it is, so I try to minimize the damage. Sorting helps too (as opposed to keeping items of a set together).