Organized black drug gangs in major cities station street dealers in a specific spot. These guys sell small quantities—single joints and small packets (sometimes match boxes) of loose marijuana. The dealer keeps a limited quantity of product on his person. When he has sold that quantity to various customers a runner delivers more product to him and collects his cash. There are many clever variations on this technique but it limits the dealer's criminal liability.
And these street corners are considered valuable proprietary ‘turf’ and are defended against rivals by any means up to murder. A lot of the gang violence is over these turf wars.
Location, location, location.