Perf12, in reading the introduction to the catalog, it would appear the gentleman who assembled the collection is deceased and the collection is being sold by the Meritus Trust, a Bermuda firm which serves the inter-generational wealth transfer needs of the ultra rich. The intro employs the phrase " acquiring important items privately through the gravitational pull of wealth..."
The items in question should be returned to the several archives from whence they were removed, assuming there is no proof of sale or trade by those archives. For some of these letters, for example lot 2133, a 3 page autograph letter by Benedict Arnold, I doubt there was a sale or trade.
Legal action - some of these were likely removed many years ago so it may be too late for criminal proceedings, but one would like to know if there were more recent removals and acquisitions. Much depends on the availability of the gentleman's acquisition records.
I am reminded of the case in 2010 when a Dutch researcher located a stolen letter by Descartes. The letter was stolen by an Italian mathematician in the 1840s from the Institut de France. Sometime in the 1800s it was acquired by an autograph collector who, unaware of the shady provenance, donated it to Haverford College in Pennsylvania. Once alerted, Haverford returned it to the Institut. The letter sheds light on Descartes thinking as he revised his Discours de la Methode (not "sur la Methode").
https://www.nytimes.com/2010/02/25/...scartes.html