Thousands of Books - ECommerce - Best Way to bulk update IDSN?

TLDR; How do you update dozens or hundreds of products / cms with daily excel csv file import Without Duplicating products or items?

An elder family member has thousands of books in his library, including autographed first editions in excelent condition, book collections, rare magazines, etc.

He currently only does local, word of mouth, craigslist and mostly cash.

He wants to start selling them online, ship USPS.

He doesn’t like Amazon or Ebay or other 3rd party Book Resellers because of fees. He only use them for his $80+ books.

He is interested in some Shopify Book Themes. He also likes some webflow themes, but none are ‘Book Store’ specific, I reassured him thats not an issue.

I showed him webflow and how easy it is to do a online store and sell products since he has all his books in excel. He likes you can upload an excel CSV file for inventory updates.

I demo Products / CMS it to him and he likes it.

But when planning it, I hit a snag.

He simply wants to upload his excel csv file every day. When I tried this before I got a bunch of duplicate entries.

Is there a solution to simply upload a file and Overwrite existing items with updated?

For example

Item 1 | Inventory 2 | Price 10
Item 2 | Inventory 1 | Price 50
Item 3 | Inventory 1 | Price 90

but now he wants to update it
Item 1 | Inventory 1 | Price 15
Item 2 | Inventory 1 | Price 70
Item 3 | Inventory 1 | Price 115

He might have 50+ books that need to update IDSN field at any given time. Etc.