None of what you shared would make sense for RMT.

If you know they're buying the materials off the MB at a certain price then selling the completed glamour item below the cost of those materials they purchased, they've lost gil not gained it. What are they supposed to be selling for real money?

It sounds more like someone who has made a lot of gil in the past now using that gil to make popular glamour available to players for less.

You're incorrect when you say an account can have a maximum of 10 retainers. An account with the standard subscription could have up to 400 retainers(40 characters with 10 retainers each) per physical data center. An account with the entry level subscription could have up to 80 retainers (8 characters with 10 retainers each).

As for why a player would want hundreds of millions of gil, that's up to them. Some players are curious to see just how rich they can get and they don't number their gil in the hundreds of millions. They number it in the billions. Personally I think it's crazy that SE allows players to accumulate that much when gil has so little value to actual game play but it is what it is.

I'm not going to deny that there are players engaging in RMT but I'm very doubtful the player you're talking about is one of them since they're selling at a loss if what you say is accurate.

You can report the player as often as you want but based on what you claim is happening, they're not doing anything wrong. They might be monopolizing certain portions of the marketboard related to glamour but they don't "own" the marketboard.