First of all, you cannot sell "inventorship," you can only sell "ownership," and the exclusionary rights under the patent.
The problem with a system where you can't sell patents is interesting. It essentially means that a patent is only worth money if you use it against somebody. And there are some problems with that...
- Solo inventors can't always afford to sue, but they usually can afford to sell.
- Some groups, like universities, hospitals or research groups, don't want to sue, or deal with lawsuits, so they'll just sell.
- Companies want to invest tons of money in research, and if they go under before they turn that research into a product, being able to sell the patent, whether in bankruptcy or because the research didn't pan out so well, really helps cushion the blow. This in turn incentivizes those companies to invest more in research.
- Eh, massive companies play war games with one another all the time. And sometimes, selling the patents to the company that wants them more actually allows more stuff to come to market. So, who cares?
That's not to say that it's a perfect system. If you wanted, you could imagine a system where you could neither sell a patent nor grant an exclusive license. That might result in a lot more permission... or a lot more trolling.