This is a highly customized version of ZenCart to allow multiple sites to use the same database. The crux of the problem was maintaining multiple shopping carts and keeping software current. So, the solution was to create "pod" sites that all read from one ZenCart database and check out through the same system.

Each pod site reads from ZenCart to locate current pricing and inventory, then when the user checks out, ZenCart presents a unique template depending on where the customer was shopping.

The big ancillary benefit of this site is that even though there are multiple stores, they can all be managed and tracked from one interface.

Here are two sites that use the same checkout system:
BuyCalmax.com
Isaderma.com