Blackmarket Guy is a guy which stands between 2 houses. You can sell items with him
Just click on him.
Available items to sell:
White Requiem Arrow for 1,000,000 money
[Limited] Pumpkin for 4,000,000 money
[Extremely Limited] Candy Corn for 18,000,000 money
The Sun for 3,000,000 money
Darkest Jotaro's Hat for 3,500,000 money