How to Get illegal Weapons/items

For Weapons

navigate to 625 back alley thean tp into the building using the circle and by pressing E

here you can obtain all illegal weapons and ammo hold left alt (third eye) and look at the window then klick left click then klick on Open Black Market

For Items

navigate to cayo periceo at the red pill blip inside go to the table hold left alt (third eye) and look at the table then klick left click then klick on Open Black Market (items)