Most of it I think is twofold; one, no easy relisting option means it's a HUGE inconvenience to list things on the Flea Market once you have a large inventory of things to look through. This is a problem for items and dogs, but obviously dogs have more kinda pressuring people to list them, what with limited space & all. Still, you'll see a lot of the same faces there too, just more of them. When I'm not swamped by projects I try to keep a few dogs listed, but not for another several weeks probably with how my current project is going. :P
Two, there is just... not a lot of demand for most things. It's so easy to be buried in heaps of resources to the point you just don't ever NEED to find someone farming a really specific thing. There's not a ton of useful resources found in limited areas, you can get a lot of the same stuff from different sources, so you end up with a bunch of anything you could possibly ever want. With the mole market, you'll have a bunch of players that have so many pebbles passively piling up with NO pebble sinks available, so they just buy out the crafting sections of the MM just to have something to spend pebbles on. I've recently started doing this myself after hearing about it, and it's not like I'm even bothering to craft a bunch of ribbons or armor sets or anything. Just hoarding it for new items nebulously released in the future I guess?
Right now, I think the most ""inconvenient"" item to get are the gems that are scattered in particular mining zones, but that's... Especially with the mining pick that prevents stamina use, it's not really a big ask to just go mine in another spot for a bit to get some gems. Most of what you even mine for is just to make picks and wheels to keep mining, so you aren't really... missing a whole huge world of income by not living in the last zone 24/7.
So instead of ever going "hm, I need a thing, let's go to the MM and buy it", it's always "well, where is this thing found, oh well that's easy to get, let's just go get it with minimal effort". Or more likely if you've been here a while, "Ah, I have 20000+ of the thing on accident, that's nice for me."