>100 players made radiant star gem
>out of 100, 70 quit and sell their gem on the market
>the remaining 30 already have the gem
>the new players are not that much
>thus, supply > demand

As for Asia server, I'm not sure since I don't play there. It could be any reason like duping; gold farmers (more gold = more gems created = market crashed where too much supply, too little demand [ASSUMING the server is flooded with bot]); or any other reasons.

For the second point, it really depends on individual.

If a person truly wants to quit, I doubt he'll care how much it'll be sold for, as long as it can be converted back to real money, since gold can be converted back to cash by rmah / any other means; regardless of the $ earned (since keeping Digital Goods will not really bring them any advantage).

There's people like me as well, holding the gems and not selling due to the low market value, hoping that it'll raise back up so that I could sell it for a profit, even though I have already quit.