Good point. Am sure we were all thinking all auction houses looked after only posters...
That said, the auction house only needs a good relationship with one shipper. End of auction, all items needing to be shipped are gathered in on corner, shipper comes into quote, auction house adds 10% to the respective invoices, everything gets shipped off to the shipper for packing and moving on.
Am sure I have simplified things and there might be a requirement for space in warehouses until everyone has paid, but... That said, I wonder if one shipper than multiple shippers stomping all over the place may be less of a headache for the auction house.