Why so many JL Audio Fathom subs for sale I wonder

Because JL Audio subwoofers were selling like hot cakes a couple of years ago and like most things in audio it is time for a change. I almost bought a JL subwoofer myself, but it didn't offer the type of connection I wanted, so I bought a REL instead.

JL is mass manufactured product just like bose or jbl.

People realizing two channel sounds better as two channel...

Honestly, I looked a few months ago an there were only a few. It is hit or miss.

Usually highly reviewed = more sales.
More sales = more re-sales.

Pretty simple math.

IME, it's difficult (tho not impossible) to properly integrate subwoofers into a 2 channel music system by ear. Many, many moons ago I tried and failed. I didn't revisit the idea for three decades. Now, I use Audyssey and it's simple and straightforward. However, I doubt that many people here want to go that route. Instead, they try manual set-up and, not surprisingly IME, are disappointed with the results and sell the subs.

As to JL specifically, I suspect it's because JL and REL share the biggest piece of the high end audio market for subwoofers. At any given time, I'd think that one or the other would dominate the re-sale listings here. OTOH, given the lower price point offerings, I'd think that you might see more Velodyne products on e-bay, or the like.

I own Rythmik subs, but I've auditioned JL extensively and think that (provided they're properly set-up) they are outstanding performers.

Marty