This can be earned easily after you are a bit into the game. I was about 60 days into the game, and then turned the difficulty from 0 to 10. I started getting robbed pretty heavily. Have one of you employees as security and they will hit the people that steal with a broom and recover the items. It might take a few days but it will come over time. You can check your progress at the Manager Blackboard, in the Statistics tab there is a section for "Stolen Products Collected." Once this is at 500 you are good.
The achievement will unlock after you close your store for the night after you recover your 500th product.