A buyout is when you buy a product yourself but get a 30% discount on that product in exchange for a review. There's no limit placed on how many reviews currently exist on the product.

Free "assignment" reviews come in two forms. You get one monthly assignment which you get to pick from. The cost of this is based on your ranking. You can pick any product in your price range that has less than three review on it (for toys) or one review (for lingerie). There's also manufacturer sponsored reviews which are usually "pushed" to the contributor from admin, though they sometimes pop up in the Product Search and can be requested. These don't have limits on price and since the manufacturer is requesting the review any pre-exisiting reviews don't count against getting the product.

All of this hinges on being an advanced reviewer, which you can become from graduating the mentor program or going up for community vote.

Hope this helps!

You can read more about the review program in this thread: The Student's guide to the Mentor Program!

The pricing guidelines for reviews are ~*>HERE<*~.