They appear legit. Probably managed to purchase an archive of letters. Hoover's signature is fairly common - though the seller is now effectively flooding the ebay market with those signed items which will likely dilute prices further in the long term.
Frogtown has a lot of nice, but extremely expensive items that appear from time to time at RRauction and others, but often they don't meet his reserved prices. I don't know about Hoover's letters, but the autographs that I know are authentic.