Selling Video - What is/is not legal?

Hello,

For the past few months I have been working up some plans for a sort of service that pertains to visual effects/cg content.

Don't want to talk about it much here, but I'm trying to evaluate the legal issues that could arise from the said service.

I essentially want to build an interactive web app that allows users to input and customize After effects templates, and have them rendered and sent to them from my home server ( render farm. )

I have been making some free templates for quite some time, And I have noticed that many users either don't know how to use the software or have some sort of hardware limitation. This web app is targeted at those users.

What I want to know if whether or not it is technically speaking legal for me to sell these videos? And also is it legal for me to say buy templates from a site like "Videohive" and have those templates customized and rendered and sent to the users?

I don't see why it would be an issue, as I'am not selling the project files nor any of its contents. ( just simply a final h.264 video clip customized to the users liking. )

I have a license for all the software that would be in use...is there anything I might be missing?

Thanks

Csaba