If you'd like to restrict access to your page or your sales, you'll just have to follow the steps below:
Go to your fundraisers settings by clicking the cog button from the Dashboard or the Page Editor.
Afterwards, select the Fundraiser Type tab. At the bottom of the screen, you'll want to toggle on This page is private.
If you're looking to only make the sales private, choose Checkouts Only, otherwise select Entire Page. Then enter a password, update your page, and that's it!