If most of the activity that requires logging in is going to happen on Discourse, I think you would be better of using the wp-discourse SSO Client option. That will allow users of your forum to easily create an account on WordPress when they want to buy something. You can embed a WordPress login link on your forum to make it really easy.
My guess is that using the SSO Provider option could decrease activity on a forum if users don’t have a good reason to be creating accounts on WordPress. (I don’t have any data to back this up though.)