This will just take time to unlock. You can make progress towards this very early by putting items for sale with the Sale Flag in the Manager Room. You just need to put items to a minimum of 5% off to count towards this achievement.
As you progress, you can get upgrades that allow you to do more sales at once. You have to put items on sale everyday, they do not carry over to the next day. You can check your progress towards this achievement on the Manger Blackboard under the Statistics section. Once you get "Sales Done To This Day" to more than 50 you are good.
The achievement will unlock after you close your store for the night after you do your 50th sale.