If i rendered a video in yaf ray can i sell it or would i have to pay yaf ray?
YafRay is Open Source, same as Blender itself. You can not ‘sell’ the software but whatever you create with it is your sole property and you can do with it as you like.
BUT, if you did make some money using it, it would be sweet of ya to pitch a little bit in the ol’ coffers of the programmers. 
Yafray is open source (as one of the above post has said) however what you have produced is a video which is classed as data and not code or software, also the yafray render engine did not insert any of its GPL code into your work so you are more than safe to assume that what you have is royality free and not under the GPL.
You can sell the software - you just have to distribute the source code along with it. This is how RedHat and other Linux companies work.
Certainly 8)
You can sell the software - you just have to distribute the source code along with it. This is how RedHat and other Linux companies work.[/quote]
You are right of course. I should have made it more clear: I meant that you can not sell a GPL-software as your own commercial product. You are alowed however, to ‘distribute it’ and ask payment for this distribution.
It can be explained in much more detail, but I think the initial question has been answered.