I want to add a new product_type which covers products, customers can rent for a period of time. Before I start I would like to hear your comments. Further more I have a few questions I hope you will answer.

Just as we have a table called "customers_basket", I will create a table called "customers_inventory".
What functions can calculate the price for a product with attributes in the table "customers_basket"? and can I use the same for my new table "customers_inventory"?
When does products leave the customers_basket in general?

How can I prevent the shopowner from deleting an attribute for a product which already is included in some customers inventory?