[5e] Any ideas for a "visiting the black market" scene?

You could also look for some cursed items and put those within their price range while putting other magic items with only beneficial properties just outside of the richest one's price range. That way, they might buy one of the cursed items. But, you could also make the cursed items have a condition that lifts the curse and turns the item into something really cool that they can use.

Example: A kriss dagger. It's curse is that once it is unsheathed, the user cannot take it's hand off of the handle of the blade until it is used to kill a creature from another plane of existence. The perk: Can cast misty step a number of times equal to your constitution modifier per day. You regain expended uses after a long rest or after you kill a creature from another plane of existence.

Half the fun of a black market scene is the atmosphere. Really play up how shady the scene is (figuratively or literally). Litter and graffiti on the street outside. There are urchins on the corner who have been paid to keep an eye out for guards--or the guards have already been paid off. The players might need a referral from an underworld contact to do business with certain people. There's some very mean-looking "private security" lurking nearby. Everyone emphasizes that they are legitimate businesspeople, in a way that no actually legitimate businessperson ever has to. This guy turns up.

Cursed items are a must, try some endangered animal hides, and try to have some kind of gang

Avoid - Rape and Slave People