I've noticed the same thing for any MPX variant these days. I have honestly considered letting go of mine in order to fund another project, just seems like a good time to flip it because it was an expensive gun to begin with and, just like you said: guys are buying/selling them at ridiculous prices. I bought 2 and tricked them out for USPSA PCC division when getting back into matches again after a ~5 year break... but since getting back into it I mostly shoot pistols again in USPSA now (I sold the one I ran hard to a buddy right before the pandemic actually, kept the identical back-up).
I wouldn't gouge anybody, but I would be ok with getting almost every last cent I put into it though with a clear conscience, and whomever bought it would still probably feel like they got a steal. I mean 41rd MPX mags were ~$100+ in the "good times" pre-covid, and I've got 6, plus it already has a Hiperfire, Raptor, better furniture, magwell, you know, a list of all the shit one might want for an MPX but that is also tough to track down or backordered forever...
Problem is, then I go shoot it.
I don't even want to think about what the current dollar amount number would be of 9mm that I can get rid of with that thing in like 5 minutes.
It's so fucking fun, I can't sell it yet. Smile and laugh inducing shredding machine every time. I've handed it to total grizzled, grumpy retired assholes who are never happy about anything, and they laugh like little girls as they're knocking down steel at warp speed. Everyone should have one.
Did I mention I hate Sig? I hate Sig, because since the early 2000's they use us to beta test their shit and they make us pay top dollar for the privilege. I'll skip over the pistols that go off by themselves. But, how the MPX works is cool, I really do wish it wasn't proprietary to Sig. If the design was "public domain" (or whatever you call what the AR design is considered) the short-gas-piston design could be even better and cheaper (probably would be just fine for Sig too).
Gun-nerd time: the weird/cool thing about the short-gas-piston (and roller-delayed too, like an MP5) vs a blowback gun if you haven't shot one, is they "eat" the recoil inside the gun, the action soaks it up. It has a really cool feel to it. All PCC's are fun because there's no concussion or blast like an AR has when you run one the same way, most don't realize it, but most AR9's kick more than an AR. Even with the really fast blowback guns, you have to load and tune your ammo (usually fast and light) to your gun and use a comp/brake sort of like a 2-handed Open gun, and even then that's to try to get the dot to track best for you, not so much for recoil feel. The MPX is just different, weirdly it loves hot factory ammo, it's almost strange at first how super soft shooting it is without doing anything. But if you reload, things can get ridiculous soft (as long as you don't go too soft to make it unreliable). I can run my USPSA pistol bunny-fart ammo (~130pf out of a Shadow) through it in the summer and it's absolutely stupid, but it chokes every mag in the colder weather unless I feed it hotter stuff, that's just how it works (lower temps, lower pressure, gamer rounds no bueno).
IDK, even if I sold it for top dollar, I don't think I could buy anything that would be the same amount of fun. I suppose I could put together a super-fun blowback rat (ala Max Leograndis type build) for a lot less money that would be almost as fun, but the thing with the MPX is that it's just on another level sort of: you can just crank out rounds like a buzzsaw, somehow it's better than full-auto, the dot doesn't move. It's so fun that I'm afraid if I get rid of it I'll regret it later, so it stays (for now).
TLDR: Yes, a lot of those guys are price-gouging assholes, but I'd like to think some of those guys are like "this is what it'd take..."
Don't get ripped off though, you'll find something fair sooner or later.