How to write a repository to get private information related to any type of product

Assuming that you are talking about "how to store an unknown number of items that are related to a particular product in a database" then the best way is definately to store the items in a separate table, with a foreign key reference to the ID of the product itself: it's how we do it for pretty much anything.
For example, take an invoice: You have a table which contains the "one off items":

ID
CustomerID   - foreign key to Customers table
Date
Total

Then each item they order gets a separate line in a separate table:

ID
InvoiceID  - foreign key to Invoices table
Quantity
ProductID  - foreign key to the Products table
PerItemValue

You can then assemble an invoice for a particular company using SQL JOINs:

SELECT L.Quantity, 
       L.PerItemValue, 
       P.Description,
       L.Quantity * L.PerItemValue AS LineValue
FROM Invoices I
JOIN InvoiceLines L ON L.InvoiceID = I.ID
JOIN Products P ON l.ProductID = P.ID

And you get a set of rows returned that look like this:

Quantity   PerItemValue  Description          LineValue
       6          19.99  Expensive plates        119.94
       2           9.99  Saucepan                 19.98
       1         199.99  Small TV                199.99

That way, it doesn't matter how many attributes you associate with a product, or what attribute types they are.

I'd put the attribute types in another table as well, and add the appropriate foreign key to that as well.

Make sense?