I made a dumb mistake. There are in fact only 67 sets available, not 75. I accidentally included some off-limit sets in my count. I emailed Mr. Fiction to work it out with him, but thought I should amend the offer for anyone else who is interested. The deal will be for 67 sets, not 75 as I mistakenly said above.
My bad.

As I said, I have contacted Mr. Fiction privately to work out how he would like to proceed.