How to sign out of the App Store on my iMac desktop?
Is not possible.
Shouldn't there be a way to see account balance without seeing whole account or at least a way to sign out of account?
Yes, just click App Store from the Dock, then you will immediately see your Name and your Apple Account balance (bottom left).
If you then click on your name (bottom left) in the App Store, you will see your purchase history.
If you click on 'View Account' (top right) next to 'Redeem gift voucher', after sign in, you will see your Account Information.
As far as i know it has always been that way.
If you want to see your Apple Account balance, see the following Apple articles:
Check your Apple Account balance
Check your Apple Account balance – Apple Support (UK)
Check your gift card balance
Check your gift card balance - Apple Support
- on your iPhone -> Tap App Store -> tap your profile (top right corner of the screen).