On your second point.... Governments should be assumed to be bad actors and they certainly have some of the deepest pockets. If an open market increases the price, it would seem to make it a better market for bad actors.

Especially if we think of small software companies or open-source projects (like OpenSSL) who cant afford to pay hundreds of thousands of dollars to secure their own exploit.

On your overall point... I think this issue of selling 0days is more a debate of ethics, and I don't think economics can solve a problem of ethics.