As far as I know Bitcoin started evolving after the huge success of Silk Road (
https://en.wikipedia.org/wiki/Silk_Road_(marketplace)).
Be extremely careful if you set a black market. The first clients can be secret service agents. They can come with hundreds of
BTC and become your friends. Few month later you can be in a prison

.
You will only be in prison if you are doing something illegal, which is why most people explore the dark web anyway. I wonder who invented it in the first place

Yep so thats why its quite dangerous selling something, they'll fake that they buy something and then before you know it your getting swatted in front of your house lol
if you're selling in illegal website than you must selling an illegal stuff and you're know the risk of selling it so there's no different between selling in a legal place or illegal because you're selling the same stuff that's illegal,both could drive you to be prisoned,and if you're selling legal stuff then why bother to sell it in illegal marketplace?
Of course not always you can end in jail. When you are selling digital goods and using the proper tools like VPN+TOR and using anonymous details and bitcoin or monero as the currency you like to get paid nothing can happen to you. For nothing to be happening to you , you need to do a bit of tweaks to TOR browser.
Here are my suggestion for you being safe.
Every time you open TOR check for browser update, and always update your OS, prefered OS is Linux for TOR.
Set your privacy and security settings to high, to disable all flash and java things related.
Type about:config, press Enter when told so and then type javascript.enabled and change it from true to false.
If you use Linux and do the above tricks to TOR , FBI needs 1 mln dollar to start an attack to find who you are and for that they have no reason too.
I don't agree with the stuff that is being sold on TOR dark marketplaces, just wanted to show that is possible to do all kind of illegal things via TOR as long as we are talking about digital goods.