Tying arrangements.
Alternative hypotheses.
wQuality control.
nSeller may be able to assure quality more cheaply than buyer.
lExample: cheap cards or paper cause machines to jam, thus harming seller’s reputation.
nCourt insists on list of approved suppliers, and Magnuson-Moss Act (1975) makes it illegal to condition warranty on use of approved complements. 
lBut tying may be low-cost way of guaranteeing performance.