https://store.steampowered.com is the official steam marketplace. It does not use a self-signed certificate.
> There are many benefits to using a self-signed SSL cert over purchasing a CA one.
I'm sure there are (not that any are immediately coming to mind), but this is the official web marketplace. It is NOT self-signed. It's signed by DigiCert, Inc, which is easy to check if you go there.
To clarify, store.steampowered.com is not some special app store subdomain. It's the official store, and where you are redirected in a browser if you go to steampowered.com. It is very unlikely they are using a self-signed certificate given it's an actual web address used by many tens of millions of people in their browsers primarily.