have vendors considered doing two sales? one for half the world; the other for the other half.
set your max purchase limits like normal, but for each sale, ignore orders from the other half.
give each half a set amount (not necessarily half - ratio is up to the seller) and if one sale doesn't completely sell out, give it to people from the other sale who didn't make the cutoff.
seems to me like the logical way to do things if the vendor is willing to do the work.
