My name is Jade, I am with the Social Care Team at Shopify.
Yes, you certainly can! Here are the steps to create a product with two variants that have the same ISBN (barcode) number. You'll see at the bottom of the instructions what it looks like in the POS app when you select which variant your customer would like to purchase.
1) In your Shopify admin click Products > Add Product.
2) Add all of your product details for the book as normal, making sure the product is available on POS.
3) Scroll down the page and add the ISBN number for the book. If you would like to keep track of product inventory select that option or just leave it as is.
4) If you're selling your product online be sure to fill in the shipping details as well. Further down the page, you'll see a section to add variants.
5) From here you can add a variant and your new and used options. I called my variant Condition with the options New and Used. You'll notice that I made the ISBN number the same for new and used variants. Click Save when you're done.
6) Now, to complete an order in Shopify POS. Open the POS app and select the product you just made. When you click on the product it will ask if you would like the new or the used book.
7) When you select a variant it will be visible underneath the product like this.
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Jade | Social Care