I feel you !! Good luck !!
As I said earlier in another post, if I were living in America, I would consider doing exactly this, having a shipping warehouse/consignor and having partnership with auctions house...
I wrote to about ten shipping companies in USA to find if they can offer such service, none replied.
So, fellow Americans, if you know any company in USA who can offer such services against fee (to cover storage, transport and insurance), it would nice to share ! 6 weeks is too short in EMP (unless you buy a lot but for people like who has a wishlist, it is too short) and yes, buying a single poster can be expensive in shipping... I would be happy with a shipment every six months