Directorik,
You raise an interesting point... however, because of the double-sided platform, there will be enough users on the purchasing side that will want to purchase your film if it is good. There will be a sophisticated algorithm to rate films on several different levels and thus drive purchases of good content. If your film is great, which it should be, people will be willing to purchase it thus creating more copies to be rented.
Also, if your film is good, and rentals are in short supply, then users will be more apt to purchase the film netting you $5 instead of the $.50 you would have made off that film if it were available for rent.
In other words, the system of allowing films to be rented only after they are purchased creates an incentive for people to take a risk on good films, thus being able to watch it themselves, and creating an additional "copy" out there for people to rent. If there are only 3 copies of your film available to rent and the trailer is good and it has good reviews, there will be a fourth person on the purchasing side of the platform that will be willing to make the investment in the film assuming that the demand will be there for them to rent it out several times. If we are correct about this, your movie will make the initial $5 of being purchased by that marginal user, plus all the revenue you will benefit from it then being rented, since supply is low and demand is high.
I hope this makes sense, and we have other mechanisms in place that will put films that have a high rating on our home page, as well as featuring users on our home page that have a high user rating. In this way, we will reward films that are truly great, and films that aren't so great will not get as many purchases.
by limiting rentals to purchases, I believe you will actually make a lot more money than you would if we only offered it for rental.
It's important to keep in mind that there are users on both sides of the platform; those users that are more apt to purchase films and try to make money from renting them out (Thus promoting your film for you on our site) and those that are there to simply spend $1 to rent a film.
Best,
Jeff