Did they list them for more than they paid you or for less?
If that other player bought your items, then you sold them. That doesn't sound like a problem.
Again, we're missing information here.
A lot of players do things for strange reasons. Some are altruists. Others want to see how long they can hold a monopoly on a part of the market. Yet others do it just to annoy other players. When you're dealing with a couple of hundred thousand players, there's bound to be few oddballs in the bunch not behaving according to "normal" reasoning.
If you come up with information that would indicate this player is selling at a profit instead of at a loss like you claim, then we'd be more inclined to believe that it's RMT. Otherwise, you've just met one of the oddballs.
700 items wouldn't take that long to update manually for someone who uses the marketboard as their endgame.
I know you tend to keep a careful eye on the suspected bots for EU. Are you seeing this individual doing the rapid, constant undercutting typical of those using bots?
There was no need to investigate the OP's claim when the claim was that the seller is operating at a loss. RMT can't operate at a gil loss when their goal is to earn a gil profit that they can sell for real money. It's possible for things to be out of the ordinary and yet still not being doing anything wrong.
Whether this individual is using a bot or other third party program to list and adjust prices is a separate issue. If you've gathered enough information to be confident that they are using a third party tool of some sort, then report them for doing that. Maybe that's why nothing has happened despite the OP's repeated reports - they're reporting the player for RMT when there's no evidence of that and the real problem is third party tool use.
It's not a matter of motive. It's a matter of evidence. For now, all we're hearing is that someone is trying to monopolize map glamours on the EU data centers by selling under the cost of the materials they are purchasing. That's not against ToS.