You are correct about his penury, Cassius—I recently read a book called Measuring America in which Jefferson figured prominently—but it was his personal library that he sold to Congress. The British had just burned the capital in the War of 1812, including the Library of Congress, and Jefferson's private collection was the largest in North America. It seemed like a perfect match—except that many in Congress grumbled at the deal, complaining that many of Jefferson's books were far too irreligious, far too politically radical (he was highly sympathetic to the bloodsoaked revolution in France), and far too unreadable; a high percentage of them were not in English. In fact, at least five of these books were copies of Lucretius, in Latin.
I don't think his letters would have been included in this sale, however.