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To pickpocket a customer after you sold them product, you simply crouch behind them and press your interact key to start the minigame.

How I setup the achievement was to get a deal close to your base (motel, sweatshop, bungalow...etc.) and use the save by your front door. That way should you mess up, you can quit and reload the save to have as many attempts as you need. It took me personally five tries to get the pickpocket timing right, but hopefully you are luckier than I was.