Hover your mouse over the amounts on your benefits for boarders page in EC management. The number that comes up is the minimum amount needed to offer the benefit. Drop 1 below that number and the benefit will be removed. You need 2x the amount for it to be displayed as a benefit when players are looking for boxes. You also need to bear in mind that boarded horses will be using the items so you will need more than 2x if you wish it to remain visible. I usually suggest maintaining 3-4 times the required amount with continued production so you don't run out.

The same store of food items will be being used by your own horses unless they are boarded in other centre's that offer free food. Selling items only gets you half the price of buying them so be aware of that if you do sell your stores, it might cost you more in the long run.