I am starting several craft shops and I would like the customer to log-in one site, able to shop in the other shops and then have one checkout and this does not matter which shop they are in.

E.G long in - to the jewellery shop; browse and get some beads
Go on to cross stitch; browse and get some thread
Go on to Tools; browse and not get anything

Pay for the thread & beads on the tools website.