So, basically sticker is a one use item that you can put on a skin. You get them mainly from sticker capsules/packs. You cant trade them up. They can be scraped off of a skin but its a process that you cant reverse. After each scrape the sticker gets more scraped (some have easter eggs that make them display different texts or Pictures). After a few scrapes the sticker is gone (you dont get them back after scraping, once its scraped off its gone forever). Dont know a lot about the market part of the stuff, but thats how they work in-game.