How can users set their own price for their products?
Assuming that you are using the Flag module (as I suggested in my answer to your prior question to "How authenticated users can choose favorite products and show them in their profile?", you could implement an extra (price) field when you define a flag. Such field can then be used to store the Product's price.

So if a user (= Store) then flags a product (to indicated "This store sells this product"), such user could use the extra field to actually indicate the price for that store.

Refer to the community documentation about Attaching fields to flags for more details about this.