If you are using the relational mod I think it would be best to have users login to the customer database to add their information. Then from there provide the links to add, modify, and delete products from the products database.

Once they login you can provide the choices on a main page for them to enter their products.

In the FAQ noted below under relational databases there are several examples, and tips and tricks for working with relational databases.

Unoffical DBMan FAQ

http://creativecomputingweb.com/dbman/index.shtml/