AH! This should be pretty easy, and you shouldn't need to copy data.
Your customer table has customer info and a unique ID for each customer (autonumbers can be useful for this). A given customer may particpate in one or more sales.
Your inventory table also should have a unique ID for each inventory item (an autonumber again is convenient).
Your sales table has sales information, and a sales ID for that particular sale. A given sale can have one and only one customer, and one or more Sold items.
A Sold Items table would contain the inventory ID of what was sold, the price it was sold at, perhaps a quantity (e.g. floor mats), the price it was sold at, and the associated Sales table ID.
So, you have three main forms;
Customers, Inventory, and Sales.
The Sales form has:
a sub form: Sold Items. After you create a sold items form, use the wizard to add the sub form to your sales form.
a combo box for the customer
The Sold Items form has:
a combo box for the inventory item.
With this approach, you shouldn't need to use a temp table, and the information you need will be automatically copied where it's necessary.
This answer is lacking in much detail, so please don't hesitate to answer any more questions.