Unless you can make a library that is considerably better than what is open-sourced you will be out of luck. But, on the ASP side there are some for-pay component libraries that make it easier for developers, so it can work, depending on the market focus.
jQuery won't be anything other than free, I expect, as John Resig, the main person behind it, seems to be very involved with open-source, since he is heavily involved with Firefox development.
There are many benefits in learning how to create your own library, depending on your interests. In the past it was almost a rite of passage for Unix programmers to write their own shells, I can see the same idea for people to write their own frameworks.
It's cliche to say you can make money on open source tools by selling support, but the guys at ExtJs do exactly that -- and that's another cross-browser Javascript library, so it's a very relevant example!
Having said that, there's only a few companies that truly just make money off support. The jQuery guys might have enough eyeballs that they could get away with it, but in general I would advise against that as a business strategy if you're starting from scratch.
No, I don't think so. Why would they be selling? Support? that's not a startup, that's a consulting business around a specific library. It might go very well but it won't scale like selling copies of a program or a monthly subscription to a service.
If you are thinking of creating a layer of code on top of jQuery that makes it better and is not free, I really doubt people will pick it up. They'll just use plain jQuery.