I've tried about everything I've found to fix this. It seems like we have a unique problem in that we can log in, see our purchases but the arrow spins. When I look in console.app I see one suspicious thing from the app store app (run console.app, then try a download and search for 'app store' -- errors have a little yellow mark next to them)
AMSURLRequest: Failed to fetch GUID schemes from bag
Which seems to be associated with app store problems but I can't find any reliable way to fix it.
It's somewhat disheartening when
@luis.serraller mentioned that even after a fresh install he still sees problem.