One approach that may generate some visibility of the issue - and possibly some action, or an investigation - would be to file a complaint with ARIN, the responsible entity fir all North American IP address allocations.
I made just such a report to ARIN shortly after this came to light. The basic argument is that the original allocation was not made to the individuals but to the community, and the individuals (and later successor organisation ARDC) managed the addresses in trust for the community. They never owned it and therefore had no ability to sell it. This interpretation of the nature of the original allocation is supported by people who were around at the time, as well as the treatment of similar allocations e.g. SATNet, but of course explicit documentation of this is thin.
The secrecy around the sale is very worrying. Keeping the negotiations and precise terms of the sale confidential is somewhat understandble, but keeping secret the idea that selling any part of the space was even possible and being contemplated is a lot harder to understand. It suggests that ARDC knew that this was not ok or at least that they did not want to allow discussion within the community. Neither reflects well on their stewardship of this shared resource.
I do not know what the outcome of the ARIN investigation will be. They have followed up with me to ask for more information, which I gave to them.
At this point, is it possible to reverse the sale? Technically it might be but it's hard to see how it could happen without expensive, lengthy and damaging legal action. And ARDC now have very deep pockets. The best that we can hope for is that ARDC becomes transparent and accountable to the amateur radio community. It is not enough to say that it's a charity regulated by the IRS because it's not the IRS' resources that it's managing, it's ours.
For ARDC to become accountable would mean changing the byelaws to have a concept of membership and an elected board. The obvious definition of membership is holders of an address allocation. Because every amateur radio operator is eligible for an allocation, this should be sufficiently representative.
73s VE3HW