Many users with many items - means two tables related with ID fields and maybe a join table
The first five links in this list will try to illustrate the principles.
Unless each user is the only user, and data is wiped between users.
You might also take a look at the invoices starter solutions that come with Filemaker 11 and 12. The "invoice data" (12) or "line items" (11) tables used in these starter solutions would sound like your "shopping basket" and also serves as the "Join" table described by DavidAnders.