https://media.wizards.com/2018/dnd/downloads/FAQv82.pdfNonmagical Items. As a general rule, mundane
equipment and treasure can’t be kept—even if listed
under a treasure subheading
The answer is correct, but that's not really the right reference. There's a list of what you can spend gold on in the player's guide, and it includes anything in the player's handbook or other allowed books, plus some scrolls and potions.