Is this real? The seller joined eBay August 2015 and has no ratings. I'd be wary...

You might want to have a look at Findegil's "Addenda and Corrigenda to The J.R.R. Tolkien Companion and Guide (2006) Vol. 1: Chronology"

and in particular look at the signature of the secretary on the letter

http://www.hammondandscull.com/addenda/chronology.html

"p. 634, add entry after that for 31 May 1965:

Second half of 1965 On Tolkien’s behalf and from his dictation, Baillie Knapheis writes a letter to a reader about The Lord of the Rings, reporting inter alia that the name of Merry’s wife was not recorded. Tolkien, however, in an added manuscript note, says that he ‘believes’ that Merry had married a sister of Fredegar Bolger, and will look into the matter (see The Peoples of Middle-earth, p. 117, n. 1."

Baillie Knapheis was Tolkien's secretary in 1965 and subsequently married Christopher Tolkien.

As far as I can tell this is the letter referred to on page 634.

It is also mentioned here

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THE SILMARILLION Anniversary Deluxe 1st PRINTING Tolkien LORD OF RINGS

262013790638

Seller: the_merp_shop

(1020)

Ended Aug 25, 2015

This item ended more than 90 days ago

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JRR TOLKIEN - History Of Middle Earth Part 3 - LIMITED Edition Of 1000 Copies.

131586901395

Seller: cartwright9363

(468)

Ended Aug 26, 2015

This item ended more than 90 days ago

Thanks for the information. That's a useful link to have :)