Complex sentence: A registry, on the other hand, is made public and is limited to the stock of a given retailer, which maintains the list for the registrant and ensures that items are removed from the list as they are purchased.
Split into simpler sentences: A registry, on the other hand, is made public, and is limited to the stock of a given retailer. The retailer will maintain the list for the registrant and make sure that items are removed from the list as they are purchased.