Selling something online, means having a payment gateway: something an e-commerce solution would typically provide for. Searching for e-commerce plugins for WordPress the Woocommerce plugin seems to be a sensible choice (also because them being an acquired part of Automattic, the company behind WordPress).
After that maybe find a solution for buyers to become “members” of the website who can log in, giving them access to the initially restricted areas of the website.
Don’t know about the unlocking of videos in a giving order. Personally I would not go there, I would just leave it up to the customer: give them some freedom of movement.