So, if a player does not have a title to anything in the game, the player has control over some items, and can sell this control? I suppose tokens in casinos also don't belong to players, but they are readily exchangeable for money.
On one hand, this may be enough to consider it gambling for money.
On the other hand, many games may expressly forbid selling in-game stuff. This is not easy to enforce, though, unless you forbid to drop and pick up items, or make in-game exchanges. A number of games uses exchange as a important "social" / "multiplayer" feature, but likely those are not slot-machine-like anyway.