Let's face it. We, as a community, bought gil. In turn, RMTs sees "cha-ching", so they advertise more. Can't really blame the buyers, though. I have BRD Nexus, and easily bought Materia off the MB with the gil I made from Dungeons back in the day. Now, Savage Might II are more expensive (60k almost), and many don't want to Spiritbond to get Materia, so gil is probably bought to buy the Materia.

We jack prices up to get 10 million gil in our pocket to say we have nothing to buy lol... We sell high, people can't afford it, so a DRG's Zodiac has Skill Speed and Parry on it as it's cheaper.

In my opinion, the RMTs are simply flooding where people buy.