First off, this does sound a bit fishy, especially since retainer names have been put in place specifically so that people cannot correlate player names, an market presences. second of all, macro crafting is completely within the rules. they gave us the macro system for a reason, and patched it to be more potent later for the same. macro and bot crafting are very different. I macro craft, I do not bot craft.

Honestly, if he does that, just buy up his extra cheap goods and sell them at a higher price. the market decides what items are worth, not a particular person. shards on my server sell at a uniform 150 gil per shard. people occasionally tank one of the shard markets, and it goes down to like 75 gil per. I have a special fund set aside to buy ~6k of those cheap shards, hold onto them for a week or two, then sell them back at 150 per once the market normalizes. its pretty easy.

TL;DR stop complaining about people, and outsmart them instead. unfair business practices can be combatted with smart business ones, and smart practices almost always win.