Coinbase (and other financial services) do a pretty good job of requiring photo id on signup; I don't know what OF procedure is, but there's definitly precedent.
However, I have a really hard time believing that if you went to a normal grocery store, bought a bag of flour that clearly says "flour" on it, had a receipt for flour, brought it home with the intention of making some bread, and then the police bust down your door and find that actually, it's pure heroin - I doubt you would be convicted. That's why the case above surprises me so much - the guy went above and beyond, by even checking the ID, if anything it's the other person who should be convicted of crime and put behind bars here.
Of course the girl voluntarily went to the site, signed up, and accepted a cash payment. She never even reported anything to the police. She was talking to a school councilor ("Mandatory Reporter") and mentioned she was earning money by sleeping with men. The school councilor was then required to refer it to police for prosecution, even though none of the parties wanted it.
The DA was happy to take it on because Cody Wilson of course is the guy responsible for making 3d printed guns popular, and he had a number of political enemies.
That all wouldn't be good enough for sex crimes though, because even if you have a passport + gov ID + a notary signed statement saying you are 18 and your own mother vouching for you, the counterparty can still be convicted of rape if it turns out you're lying about your age.
Of course this is really all just to keep the law abiding law abiding. Criminals on the dark web will just buy a forged electronic id package and KYC is bypassed.