Sharing customers.

I have four seperate installs of zen-cart on one domain each featuring a different vendor.

Is there a mod or hack that will allow the customer table to be shared and syncronized?

There's no inherent way to share only customers. You could have one DB for all four stores, each store then shares all the same data. Then, customize the display with different file configurations for each (e.g different templates, customize catalog display etc). However then all the stores would also share order numbers, the same admin, etc.

No way this can work as each vendor utilizes different shipping methods unless there is a work around for this problem.

Only work around would be to maintain separate DB's and write custom code to synchronize customer accounts between them. That would involve retooling how Zen Cart creates and edits accounts, and then building and maintaining a "key" database that would match the customers_id for each person across all the stores (not gonna be the same customers_id in each database). It's hefty work.

I think I have worked out the solution. This is a membership buying club. To register the customer has to register with the buying club. This will be the master database.

Registration will turned off in each instance of Zen-Cart and the databases will be synchronized with the buying club database.

That's pretty much what I was suggesting in my last post. Requires some effort to implement, but doable.