People use three words, dark web, deep web and darknet market, and they mean three different things, so it is worth separating them before anything else.
The deep web
The deep web is everything on the internet that a search engine has not indexed. Your bank's login page, a private forum, a database behind a form. None of it is hidden on purpose. It is just not crawled, because it sits behind a login or a query. The deep web is the largest part of the internet and it is completely boring. Your email is in the deep web. So is your photo album. Nothing about the deep web requires special software to reach, and nothing about it is anonymous.
The dark web
The dark web is the small part of the deep web that is reachable only through a
specific network, and the one people mean when they say it. In practice that network is
Tor, and the addresses that live on it end in .onion. A dark web address does not
resolve in a normal browser, because it is not in the ordinary domain system at all.
You need the Tor Browser, which routes your connection through a chain of relays before
it reaches the destination, and that routing is what makes the address both private and
hard to find. The dark web is a fraction of the deep web, and it is the only part that
needs a special browser to open.
A darknet market
A darknet market is a storefront that lives on the dark web. It is a shop, with categories, listings, prices, a cart and a checkout, except that the address is an onion and the payment is a coin. The goods range from legal, like books and gift cards, to the things most people buy there, which is why the term carries a weight it would not on a normal shopping site. The defining feature is not what is sold. It is how you get there and how you pay. You reach it through Tor, you pay in a coin, and the transaction leaves a much smaller trail than the same purchase on a mainstream store. That combination is the point, and it is also the reason the security habits matter more here than anywhere else you shop.
What a buyer actually gets
In exchange for the extra steps, a buyer gets three things. Privacy of the purchase, because neither the address nor the payment sits in the clear. Access to goods that may not be available, or may be more expensive, through ordinary channels. And a layer of protection in the form of escrow, where the payment is held until the transaction is complete rather than gone the moment you click buy. None of those three is automatic. Each one depends on the market being run well and on the buyer doing the basic security habits correctly. A darknet market is a tool with a learning curve, and the buyers who get value from it are the ones who respect that curve.