Retconning equipment purchases is a very important thing for a good game. As you the player aren’t a professional hero, you’re just playing one in a game. So there are things characters would have taken because they aren’t stupid.

I think Blades in the Dark has mechanics for this on magic items. You just spend the gold via retcon.

For magical items, you can do that too, or dice roll for a very specific and unlikely case, such as bringing a potion of under water breathing into the desert. But the same roll would be much easier if your adventure was involving a boat ride.