I played around with the LNBits Nostr Marketplace NIP-15 plugin. It works and is overall pretty cool, but it may not yet be ready for the masses. That said, if you have ever setup an online store with something like PrestaShop or another competing product, it's honestly not all that much harder.

I created a marketplace. I created and listed a product. I sold my test product to myself. I was DMed an invoice by the marketplace. I paid the invoice. I then marked the item as shipped as the marketplace seller.

All of this will get much smoother and the entire user experience will be much better once NIP-15 events are supported across existing clients. Right now, it's pretty ugly.