Can An End User Purchase Central Station Monitoring?

Good question. I don't think it's generally illegal, but local laws may differ.

There are several services easily available online, but whether they are running their own central station or reselling others is difficult to determine. But it's not hard as an end user to buy your own monitoring.

For instance, a few seconds of Google brings up: Home Security Store, Alarm Relay, Smarthome.com, etc.

I second Ethan.

Proxy end-user monitoring purchases happen from DIY platforms like Simplisafe (They use COPS) and LiveWatch (Using Moni).

I would say limitation against direct public resale has more to do with the economics of one-off billing every month versus batch purchasing from an installer, but I'm curious to see replies too. 

Yes, lots of central stations cater to that market.

It's not just for self-install, it is also for people that buy a house that came with an alarm system (as long as it wasn't locked out by the installer).

You might need a license, depending on your municipality, but those are easy to get. Generally speaking, the subscriber is the one to apply and pay for the license anyway, not the installing company. And once the subscriber gets the license, the monitoring is as legal as any other. 

In places that don't require licensing, any homeowner can pay for alarm monitoring, and pay something like $8-20 a month. The trick is, of course, getting the panel to communicate properly. Some monitoring companies sell takeover kits with preprogrammed communicators, but some panels are harder to takeover than others.