Why do you need a username and password though? You could simply just provide the product and the price. Then email that user when the price changes. Offer a one time unsubscribe link in that email if they wish to not receive further updates.

Think newsletter with product price changes.

Which online stores? All of them?

You could wait to collect the email address until after users have searched for a product and found something they want an alert on. That way people know what they're getting into and it makes more sense why you need their email address.