I work at blockbuster. Let me just clear up how that rent it like it buy it thing works
First you can only buy the previously viewed dvd if they actually are selling it, which in my store they don't until 1-3 months after its initial release
Second you can only buy it if during the time that the rent it like it buy it sign is up on the shelf of that movie, if you missed that time frame you're out of luck.
Third there are very few copies if any that go out for sale previously viewed during that time frame, so if you miss it, too bad.
On another note when you're purchasing those buy 2 get 1 free movies it will deduct the price of the lowest priced movie.
So if you're planning on buying 4-5 movies make sure to ask them to ring it up separately cuz if you have 3 $14.99 movies and 1 $9.99 movie you'll want the most expensive movie to be free