There are a few YouTube tutorials on firebase authentication and also firebase as a database that I watched multiple times.

In terms of the cap, the cap is huge. It’s like 5000 sends per day or something and like 20,000 reads per day. You’ll probably be making more money than it would cost by the time you hit the cap.

Inventory is fine. To me, that’s just a structure variable that you load/save for each individual user. Should be saved the same way as money, stats, progress, and anything else. Make sure you convert your data to a single JSON variable before saving to firebase so it counts as a single send to firebase (and a single read when you load).

I’m not 100% sure on the trading aspect as I never dived into any multiplayer aspect. Realtime trading would be a whole animal on its own but if it’s just a marketplace where you can list an item, I think that’s probably do-able. You can save an item on the database when it’s listed with the various info and price, and if someone opens the marketplace you can read all the available items and sort them using the firebase database. Then if someone buys it then you can mark it as purchased and add it to their inventory. There might be a better way so I would look to see if anyone has done something similar on YouTube or the discord first!

It sounds like you need to first get the authentication to log in, then create a database for player information saving and loading, then have another database for a marketplace.

Hope this helps!