There are two different standards for marketplace right now, each one has its own unique feature:

nip15 based shops and nip-99 based items.

nip15 is merchant-specific shops that lnbits currently integrates with.

nip-99 is for classified items similar to Craigslist

Examples for nip-99:
https://shopstr.store and
Amethyst app’s marketplace feature.

For nip15 type of shops, you can get a list of all published merchant here:
https://nostr-markets.pages.dev
Click each merchant will bring you to the list of goods that specific to that merchant.

TLDR:
For shop owners or hobbyist that casually sells goods, you probably want to use nip-15.
For people don’t need a dedicated shop for accounting purposes etc you may just use nip-99 for easy of usage.