This mission is unlike anything else you've done to this point. Get into Mr. Whoopee and start to roam the neighborhoods. Turn on your ice cream music (L3) and stop, and if any junkies are nearby, they'll flock to your vehicle (thye'll be represented by yellow dots on the map). You'll have to wait until they complete their transaction in order to get credit for the sale.
You will gain one wanted star each time you make four transactions, whether or not a police officer sees you.
The best place to sell your drugs is on the main strip that runs in front of the Ocean Beach hotel (your starting safe house), between Ocean Beach and Washington Beach. There are no gang members here, and there is plenty of room to evade a wanted star after each set of four transactions. Once you gain a wanted star, start moving, even if it means aborting current deals. When you reach the south end of the strip, turn north and continue. Similarly, when you reach the northern end of the strip (where it begins to curve east), turn around again. You will rack up 50 sells with a minimum of fuss,
Depending what section of town you're in, each transaction will yield different amounts of cash. Starfish Island is most lucrative, while Little Haiti and Little Havana are least so. The average amount per buy is $10.
You'll have to string 50 sales together in one mission in order to succesfully complete this challenge. When you do, however, the Cherry Popper Ice Cream Factory will begin to make money, to the tune of $3000 a day. You'll also get $3000 for completing the mission.