Star Coins don't seem to expire when they're being regularly used, so I agree either a global countdown is being reset, or the oldest coins are being used first (indistinguishable without someone going out of their way to make some coins expire).

I wouldn't call this space whaling either without more information. Let's just assume you buy 9800 Star Coins every time you want to get an SSR KH and an SSR Eidolon guaranteed. That leaves 3800 Star Coins left over. Over a year, or twelve months, that comes out to 45,600 Star Coins. So really, the only unusual thing we know for sure is that he doesn't use his leftover Star Coins to buy other things, and just buys a new set every time. If you think just buying guaranteed SSR's is space whaling, you are sorely mistaken.