I have market press activated on multisite. When I add things to the cart they show up, but disappear when I check out. It is working fine on the sub-sites ex: http://kisses.iholdlovewithyou.com/store/products/
(Though there is a warning tab that shows up in the firefox browser.)
On the main install products disappear when checking out. Ex: http://divinitree.com/communitystore/products/.