Well... that's exactly what several of us have been saying off and on for several years now. Lynn talks about it every now and then, when he isn't out on his beach, or whatever it is they do in Mexico between bullfights. Serious collectors, folks who want everything they can find on dining cars, or skiing, or the First World War, are a huge source of potential income.
The bookselling business has always been divided into those who have serious collector customers and those who do not. In the past, those who did not tended to sell a lot of material to those who did, and many of them made a darn good living at it, though they also worked very hard to do it. They were "pickers", or general dealers, and many of them cultivated specialist dealers, who they would call when something interesting came in in their field. The specialist dealers then turned around and sold the item to their serious collector customers.
Then along came the internet, and the book databases, and the idea that you could make a living selling single books to one-time-only customers, which is fine, but it only works if the internet isn't glutted with books priced cheaper than yours.
The other idea that came along with the internet is that it would kill off the specialists, because all the serious collectors would be able to buy from anybody who had the material. The pickers and general dealers were going to be able to sell directly to the collectors, and many accordingly raised their prices and stopped calling in the specialist dealers when they got something good. Specialist dealers, as the "middlemen" in the chain, were expected to go belly up.
Except it did not work out that way.
Although many serious collectors have found their way to the internet, many have not, and have little wish to. The fact is that it takes a lot of time to ferret out good obscure stuff on the internet, and there's a lot of dross to sift through in getting there. Many serious collectors, while they enjoy going to book fairs and ephemera shows, don't get the same joy out of sitting in front of a computer screen scrolling through page after page of misdescribed, overpriced crap.
Which is where the specialist steps in- the specialist, as he always has, acts as a filter, spending his time scrolling through page after page of misdescribed, overpriced crap, to find the gems that he needs to feed the collectors.
So, yes -serious collectors are a good source of income. How does one find those collectors? It takes time and money (often lots of money) -it takes advertising and otherwise promoting yourself, some of which is free, and some of which is not, and remember that your time is money too.
What does any of this have to do with Jean Arthur? Well- I'll bet there are some Jean Arthur collectors out there...