It's a legit move on their part, but it's still bad. Legit, but bad. They have no way of automatically detecting RMT guys, so they have a check instead. OP said he made a fortune from the market, that means large amount of gil being transferred to him, which raised a flag, and auto-ban. It's not about the amount of gil you have, it's about how much you got and how you got it.

Aion did it too before, banning anyone who gets mailed with a lot of gold. People even grief others by sending gold to people they hate to get them banned. Pretty sure other MMO did the same, how else they're going to detect gold buyers?

Sadly, this is the easy way out. Flag check and auto-ban is lazy. At least they could keep the flag check, but investigate before banning. Someone getting 100k from selling Lominsan Anchovy is clearly suspicious, but someone making 150k from selling HQ rare items that everybody wants seems legit.