I wish you could either sell back, or "re-gift" unused sleeves of balls. From time to time opponents gift me balls at the end of a match, and they're not always the balls I play on a regular basis. If the specs are close, I use them somewhere eventually, but if they're wildly different from what I'm used to, they usually just sit there unused. Would be nice to either sell (or trade) these back to the pro shop for balls I'll actually use, or re-gift them to people who will use them.
As far as I can tell, WGT doesn't get screwed if they allow this; the balls in question have been purchased - it's not unfair that they be used by somebody. And once they're used up, they're gone, and more balls get purchased. Where's the downside?