I think those boards in decent condition go for somewhere near $1k. You can find Craig Kelly Air boards selling for $300 but this is a rarer find. If you do sell it for money, you should probably let your friend in on it. You definitely took advantage of him. If he doesn't care then fine, but I know if I found out about someone taking advantage of me like that, they would not be my friend anymore.
Then again, maybe this is a friend you don't need...