Is it possible to use Construct 3 to make a multiplayer game like this?

I would like to make a game in which players own their store. Each player can visit another player's store (even when that player is offline), to view the items in the store and even make changes to the items in the store. Some items in the store will be user media (photos uploaded by the users).

I think it may require Firebase to store the user media and the store info. However, I'm not sure if it is really possible to build as I'm not familiar with coding. Could anyone give me a hint about it?

Yes, it's definitely possible.

But third party servers and interfacing with them are generally beyond the average forum goers knowledge here. Best familiarize yourself with the documentation and work from that.

To simplify it, the game you're talking about basically have a function read/write data to server. Construct can already do that, to retrieve a JSON data from a server. However, you need to do some backend side programming to achieve what you want.