Some people really see the marketboard as a pvp arena and they don't even care about making money because they have so much at this point that it's about more than profit for them. They want to sell first, and that's it; and if you undercut them by 1 gil, they'll "teach you a lesson" by just crashing the market for fun. This can get really nasty when you're selling crafted items that have your username attached to the bottom of it, because people can see exactly who's undercutting them and take their anger beyond the market space. While I've never dealt with this personally outside of occasional undercutting, it doesn't surprise me to see this occur and I've seen people sort of talk about doing it more than once (the "crash market" thing, not the bully thing). Sometimes the only "aggressive act" you need to engage in to gain someone's aggro is to compete for the same market share. It's legit possible that all OP did was "dare" to sell something that a mentally ill market-pvp champ also wanted to sell.

Anyway, good luck out there. Some people in this game are really, uhhh, "Special" lmao.