The problem with this is that the stickers and capsules are only worth anything whilst the game are still being played. As soon as a new version of the game gets released or replaced by some super cool game, the price will plummet. We saw this happening with games like Starcraft I and also Broodwars etc.
Not only that but every other risk associated with centralized/monopolized market applies here as well. Valve could go bankrupt, get hacked, shut their servers down for any number of reasons, become greedy and implement "property taxes" in addition to the commission. On the other hand, if they were truly benevolent they could find a way to "migrate" those items into a new version of the game if it comes to that. Regardless, investing into something that 100% depends on one company's whim sounds like an extremely bad idea. Obviously a lot of people don't think so.