Earlier quoted context omitted.
Not to mention that trading/selling items is something that - within individual games - has been done for decades now. Diablo II anyone?
Yes, you can build all these things yourself. That's not the question (and it's what I was expanding on in the comment). The question is should you, given that there is an existing layer you can build on top of that already handles that?
From there, implementing "Player X has given item Y to Player Z" is pretty straightforward. It's even built into most RPG multiplayer games by default, unbound to any form of external payment.
Multiplayer games also add event logs - creating a immutable record of all inventory changes - just so customer service (CS) can do their job. For example, in Everquest, GMs (aka CS) were able to see that yes, indeed, a player did get the drop from the dragon, even though it later glitched out of their inventory, and re-gifted the item to the player.
So, for a single game, a blockchain only adds a second, delayed (and relatively expensive, since it requires extra compute resources to create) record of events that are already recorded.