Holding the C key was/is the command to boot to a CD or DVD. What the seller was supposed to do is put the original OS back on the Mac before selling it, not leave it blank (how are you supposed to know it's actually functional when it arrives)? Like a lot of folks though, he probably tossed or lost the gray, machine specific disks long ago.
Here's another issue. Where did you get Mavericks from? Without waiting for an answer, just by the fact that you paid for it, I can tell you already it's an illegal copy. Mavericks was free when it was available, and was only available from Apple. Anything else is a risk to use since you have no way of knowing what else may be on that USB stick. That is, the installer may have been modified to also install a keylogger or other nasty malware along with the OS.
But to at least get it running, plug in the USB drive and start the Mac up. Immediately hold the Option key. If the USB stick was properly made, it will show up as a device you can boot to. Choose it to startup the Mac to and install OS X.
Edit: Also just occurred to me. Mavericks will not run on a 2007 Mac Pro. The newest version of OS X that will run on that model is Lion, 10.7.x. To get anywhere, you're going to have to contact Apple. Tell them you need a copy of the original gray disks that Mac came with. They'll ask for the serial number to ensure you get the correct disks. Once they arrive, you can install OS X. I think what you will get is Tiger, 10.4.x.
Or, you can skip the older versions of OS X and actually spend less by purchasing Snow Leopard from Apple for $20. Install that, which will be 10.6.3. Then apply the free 10.6.8 Combo Updater. Once that's installed, you'll have the App Store on your Mac. You can then create your own App Store account and purchase Lion, 10.7.x. That's as far as you're going to be able to go with that Mac.
