Can I buy it ?

To my knowledge, nobody sells our system. You will have to build your own. It is made of widely available parts. Plans and code are available in our CVS repository.

Can I sell it ?

Sure. The GPL licence allows you to build and sell Paparazzis as long as you publish your modifications/improvements.

Won't this just help terrorists ?

No. Please stop asking. They could go buy complete autopilots and UAVs. Why would they bother building their own?