BK, I didn't weigh in last night, because everyone was offering good advice. My only other thoughts are less pleasant:
1. once you give a gift, it's out of your control; it's the receiver's decision on what to do with it, and we do not always keep what we are given.
2. they certainly didn't sell the books because they needed the money; I know from selling books during the bed bug days that booksfetch much less than the buyer wll resell them for.
3. I've had acquaintances, several DRs on this site or no longer on this site, who've never thanked me for a book, CD, DVD, whatever, or acknowledged that hey've read, watched, or listened to it; in some cases, I know it's an oversight and in others, maybe not, but that's life. I also know that, as a gift receiver who's been guilty of all the things I've mentioned, some things things get lost in life's hurlyburly.
So, if the recipients gave away or sold something you gave them, well, of course your feelings are hurt, but perhaps they never expected you to discover their actions. They might even be embarrassed that you did. I wouldn't demand they cease and desist whatever you did for the kid's act. I'd demand the billing size be doubled and my name spelled correctly.