Check multiple values in a select statement

Given a table with sales like this:

| PRODUCT_ID | USER_ID |
|------------|---------|
|          1 |       1 |
|          2 |       1 |
|          3 |       1 |
|          3 |       2 |
|          4 |       2 |
|          1 |       3 |
|          3 |       3 |

And a table with products and manufacturers like this:

| PRODUCT_ID | MANUFACTURER_ID |
|------------|-----------------|
|          1 |               1 |
|          2 |               1 |
|          3 |               2 |
|          4 |               2 |

How can I find the users that have bought all products from a manufacturer? I know I can use

SELECT *
FROM product
WHERE MANUFACTURER_ID = x

to find which products belong to a manufacturer x, but I don't have any idea as to how to continue from there.