Is there a way to provide Read/Analyze access in Google Analytics to an outside marketing firm, without letting them see the Conversions Tab and our sales data?
It's possible but quite time consuming to achieve.
One way to do this would be to set up a duplicate property in Google Analytics that doesn't have goals/e-commerce tracking enabled. Then, provide the external marketing team read-only access to that property.
Without e-commerce tracking (or goals) configured, they won't be able to see any of your sales data.
It's a bit of a pain, but it's probably the best way to secure your data based on your question.