Web content that is accessible through "darknets," or overlay networks that demand particular software, permissions, or configurations to access, is collectively referred to as the "dark web." Though there are many of these darknets, the biggest ones are Interplanetary File System (IPFS), Tor, I2P, Freenet, GNUnet, ZeroNet, Lokinet, and Urbit.
You may be referring to darknet marketplaces when you talk about locating goods that are prohibited from being sold. These are the websites where people sell things like cards, drugs, and hacked accounts. Purchased using a bank account or credit card that has been stolen or compromised, carded goods are then resold on websites like darknet markets. They consist of apparel, software, and jewelry.
There are other websites that claim to sell goods but are frauds. I can immediately think of a few: bitcoin doubler scripts, hitman sites, fixed matches, and purported red rooms. Many scam sites can be discovered on the Hidden Wiki or other link lists, and I've grown increasingly dubious about the majority of items offered for sale on Tor over time.
Regarding how these goods are purchased before being sold on the dark web, many so-called vendors either manufacture their own drugs or purchase them from a third party and then resell them there. For example, in the case of cannabis, they might cultivate the plant in a lab and resell it on the darknet market links, or in the case of ecstasy/molly, they might manufacture the drugs in a lab and resell them. However, the electronic component and the production component might be carried out in separate places.