Can we tell whether our posts actually bring in sales?

Yes, for the routes we can see. Every share link records the channel it came from, so a Facebook post that produced four sales looks different from one that produced none.

Where it honestly can't tell

Somebody taps your link on a phone at lunch and buys on a laptop that evening. We record that as unknown rather than guessing, and the unknowns get their own column instead of being spread across the channels to make the totals look complete.

A guess stops looking like a guess by the time somebody else reads the report. We'd rather show you the gap.

What's never recorded

Who clicked. A share link carries the route and nothing about the person, because these end up posted in public and they give away no name and no address.