If you jailbreak your iPhone can you buy stuff from the App Store?

Yes, when you jailbreak your iphone it can still do everything it could before the jailbreak. A jailbroken phone only allows you to do more. You will have a second app store known as Cydia that allows downloading of apps that are not approved by Apple for various reasons. These include apps to change themes and fonts on your iphone. Also apps for wifi tethering are available.