DNMs exist for reputation and escrow. It also tends to have lower prices because suppliers list there directly, whereas your private chat vendor may be a reseller.

Also, when vendors compete alongside each other's listings on the same platform it holds them accountable somewhat to providing good quality (lest they get a bad review)

If you already trust a supplier, then private chat apps are great! But if that supplier ever tries to fleece you, there is no recourse other than to stop ordering from them.

Agree that darknets are greatly flawed in many ways. They are always being raided and taken down, for starters.

However, each market that falls is a lesson in a new way NOT to build and operate a market.

People clearly want free markets and it is only a matter time before there exists an architecture that enables a DNM that cannot be taken down and protects all users, all while providing better UX than eBay.

When such a market does emerge, I just really hope they allow BTC/LN. And the fact that currently, BTC use is trending down on DNMs and none accept LN is not a good signal.