3.5 x 5.25 looks like one of the larger rollfilm formats like 122 that was common in folding cameras.
It might be worth doing a bit of research before selling anything. As Kino said, you might actually hold something of value to archivists (or not, but it's worth knowing). Think about the kind of distribution you would like to see for these. Do you want them to be open for the "greater good" or do you just want to make a quick buck?
There are people in academia who research the history of pornography (look up a few titles from an academic library); the very fact that you hold both the negatives and the prints could be of some value.