All I know is you can't add to your own collection, it has to be from someone else. And there are occasionally major drills quests that involve this. Does it have to be from a friend? Not to be a cynic, but is there some way to trade these things with some guarantee other than actual friendship? Because some of those stickers are quite expensive...

I've stuck several stickers into albums of complete strangers. Its mostly a game of trust when it comes to stickers

The users who run this thread will help sticker people. It's pretty hard for someone to scam you in general on Subeta if you post about your transactions in the forums since there's a public record. You can file a ticket to alert staff in the event that the worst does happen.

Oh, good to hear, thanks :)

It's also fun to find a sticker buddy. You both post stickers you want in your wishlists then you sticker each others albums. You can also sticker-bomb each other at every event. Rare/expensive stickers, like the letter K, you'll likely have to get for yourself and have someone on the Collaboration Sticking Board put in your album.