I may not be who you are seeking, but I was able to tie two different domains using the same cart database to show items. It was a bit of work to accomplish but I was able to with the exception of one page, for which I hired a php programmer to help me with.
At any rate, the site that does not host the cart but does use the cart product (database from the other domain) of my own design is at:
when using the buy button it goes to the actual cart check out at
This is probably not what you are asking for, but thought I'd share how to use the cart db with another site product presentation.