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.
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.