They are built on top of those technologies, but the user isn't exposed to any of the technical complexities. Seriously, go to openbazaar.org and download it, see for yourself.
No, I'm not going to install software just for accessing a marketplace. That's just silly, because now I have to worry about compatibility with my OS and a wealth of other problems that comes with running applications natively (e.g. is it compromised by a 0-day, since I doubt it gets as much attention as one of the big browsers)