Capitalizing on a mistake is good business. I don't see how he screwed his friends, when it's the seller's responsibility to be accountable for pricing. Sorry Chikamatsu, but I gotta side with Rinomaru on this. Never make business personal, or you're out of business.
I wonder, if it was the other way around and Rino would've added a few 0s when buying (if it was a trade) if the seller was going to be kind enough and reimburse him for his mistake?