Faster Pusher Man! Sell! Sell!
Go find Lester and he'll ask you to help him push some merchandise while he recovers from a hangover. Three potential buyers show up on your GPS so set the nearest one as your destination and go to him. The deal doesn't unfold the same way as other deals, though it's still on the touchscreen. You can offload just one bag of merchandise at a time before having to drive off and find a new buyer.
Soon, rival sellers start to show up and try to outsell you. You need to offload your supply before the rivals' meter (on your top screen) is filled by their successful sales. You can try to interrupt their sales by bombing them, but you're better off just focusing on getting to the most densely packed clusters of buyers and spreading your goods.
You can actually make the sales fastest if you're on a motorcycle, as the buyers will approach you and quickly initiate the trading mini-game. If you're in a car, the buyer will have to get into the car before exchanging funds. Either way, you don't need to leave your vehicle to make the transaction.
When you've sold all of your stuff, a new destination appears on your GPS. Drive to the destination, expecting rival sellers to try and shoot you on your way. Make it to the drop-off point and you'll complete the mission.