I think the last thing you should be worried about is probably the OS you are going to be using for your servers. Being that most ISPs probably use linux in most of their servers.
Providing internet to other people with your own "end-user account" when earning money for it isn't legal as far I know... Not that it would be fast enough either.
Read some about small ISPs/howtos:
http://www.ehow.com/about_4616216_creating-own-isp.html
http://www.alliancedatacom.com/isp/g...ewbieguide.asp
etc.
And after that, you should go to a phone company, and talk about getting a backbone internet connection. They'll probably introduce you to the other required hardware and according to your choice, maybe giving you some software alternatives too.
Anyway, as any other business, sounds like you need some money for starters...

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Yeah, and I'm saving that money from a part-time job at a garden center (I'm still 17, and I'm saving up that money in a bank account). Anyhow, is there such a thing as open source BBS software? This is apparently what this thread is about. If not, I would be glad to start a new project (using probably my C++ book and a Wikiversity account).

As for the commercial grade adapter, let me tell you this...

Most any Internet connection, whether it's fiber-optic, dial-up, wireless, or cellular, has to be provided by an ISP and in most cases is illegal to tap into and make a profit off of... except for powerline Internet. This kind of connection is actually provided by the government directly through the power grid itself, and there are several adapters (D-Link is one company who makes them) that allow you to access this free Internet connection. In most cases it is kind of slow... but here's the big gig: I am thinking of designing my own commercial-scale powerline adapter that has a built-in transformer designed to boost the voltage and therefore provide fast Internet on a commercial scale (and then use 802.11AC or 802.11AD wireless access points to distribute the connection). This way, I am not pirating another ISP's connection for monetary gain... the illegal thing to do that you're talking about.

As for the bbs (bulletin board systems, really?) here's what I got from one google search...
http://en.wikipedia.org/wiki/List_of_BBS_software
http://www.linux.org/apps/all/Daemons/BBS.html
There was at least one released under GPL. I'm sure you can find a lot more.

I don't know that much about internet connections as to confirm what you're saying, but as long as you're sure I guess you have a good business chance.
However I'd "design" that adapter (not that that sounds easy either) before trying to market it... but that's just me.

Anyway, go ahead and give it a shot. It'll probably be a very educational experience anyway.