I think the entire point of them is to get us to actively participate. Being able to sell them would entirely defeat their purpose. It's a bit annoying to have to earn them so slowly, but on the plus side some of the items you get with them are NOT going to deflate like crazy, because the MP will not be overflowing with them due to limited wing stamps. I like it. Then again, it's probably because I have a small hoard over five different accounts, so I can afford all the items right now.
I have a love/hate relationship with the soulbound wing stickers, heh.
While I understand why they're soulbound, I get frustrated when I want to help out a friend get an item from the Treehouse.
This is how I feel as well. It would be nice to be able to send a friend 3-4 when they're just a little short but I understand why they're soulbound. At least the times don't circulate out of the shop, so you can keep working toward earning enough stickers over long periods of time...
I don't mind really. If I needed a tree house item that badly and didn't have enough stickers, I could always buy it on the MP.
I don't think I have ever spent any of my stickers on my main, actually. I collect them on a mule and spend them there. Of course, I can't consolidate them, but I've never really had a problem getting enough stickers for what I want.
I'm upset to see that I don't have enough wing stickers to get the Angelic Sky, but all I have to do is wait for the next event to collect more. *sigh* I wish there is a better way for me to get it faster than collecting more Wing Stickers, but sadly, I have no choice but to wait. crying
Edit: I don't mind that the Wing Stickers are soulbound. I'm still saving up anyway.