It all depends if you can get your gems sold.
If you buy all his gems and you can get only half sold, then dont buy em obviously.
If you make profit on buying then reselling his gems, just keep making money of this guy.
I have crashed glyph market few times. People who post with addons keeps still undercutting and you can grab few cheap glyph from there. Also some players may leave from that market because it doesn't profit so much anymore.
Yeah... someone crashed the glyph on my server to about 20g each (5 glyphs of each kind and unlimited supply).... There is no profit in that if I have to buy my herbs off of AH and not worth the time to farm. It's been like that for a month now. People who bots are bad, but ppl who bot and ruin multiple markets for everyone else is worse.