It would be a little complex to have object purchasing in games by yourself, but possible with third party services. However, most stores, like Steam, will not allow you to process payments directly. In that case you'd have to use their store DLC features. For Steam what I have seen is that some games with a large number of items will package them together as DLC. So you could buy a medieval weapons package for like $10, that includes 30 weapons. Or a modern fashion package with like 10 different clothing outfits, or whatever makes sense for your game.
In terms of multiplayer, there are low level networking features in Godot and peer-to-peer is possible. I don't know what the limit is on number of players, but it can't be too large. For example, making an MMO or Second Life clone that was all peer-to-peer (while in theory possible) would probably be too difficult and cost more money in development than renting a server. For a small game, like a multiplayer shooter it may work (e.g. 4 on 4) but for an open world it would likely not scale. And even then, you'd still need a matchmaking server or lobby to connect the clients. So you can't avoid it.
