Okay so I just learned of these types of buyouts in my commercial class. Non Union Commercials offer buyouts to use the footage for a set amount of time. They could offer you $500 for a one year buyout, so they can use the footage for a year, (you get no residuals), and if they decide to use it for more time after that year is over they will pay you more money.
Now I saw a breakdown today for a Project Runway Promo. I love the show so I got excited to see it and I was definitely going to submit for it. They were offering a session fee and another fee if the promo aired however the buyout is in perpetuity. That means that they will pay you the buyout fee HOWEVER they can use this footage for the rest of your life and you get no residuals. Now you have to be very careful when you are dealing with in perpetuity buyouts because they can cause some conflicts if you try to do commercials for related products later on in your life and you never know what type of commercial deals you may get offered. So just use your better judgment, try to think long term!
The only reason I would think about doing a commercial with an in perpetuity buyout is if the product is so unique that I know it's not possible that I would run into any conflicts later on down the line.