Secondarily to the primary issue of "everything desirable costs ms and there are very few free ways to generate them", is the relatively tiny player base. In a more healthy game economy, there would be a lot more competition to get things to move, and there would be a more dynamic market.
There isn't a lot of competition in the ms seller's market just because there are not many active people here; so it's also way easier for someone to manipulate without spending a ton of time doing so... which is why we see the rates climbing so much faster than on other similar games. Similarly, many people don't mind spending a silly amount of pebbles on ms because what else are they gonna use the pebbles on?
It does harm player retention, especially with new players, so it's unfortunate. But it's not something you can convince people nicely to avoid doing, you have to design a system that's robust against it. :/
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I'm not personally impacted by it, I don't care about CA & so don't buy ms from the flea market (or buy them
from the site either, lol) at all and just get my upgrades
through the free route, but I think it's a shame that's this is where we are with the economy already.