Sounds like you're making a tycoon type game. Very interesting
-Free samples of different items (cost on your part but increases the likelihood of people buying the product (can have a type of shopper that is more spontaneous and will be influenced by free samples more often)
-Sales on certain products (if you put a product on sale it's because you've noticed that people haven't been buying it so you can set a percentage for the discount and hopefully you can at least break even on your stock purchases)
-Different times of year mean different products are going to sell more. Toys are more popular during Christmastime, chocolates more popular around Halloween and Valentines, turkeys are popular around Thanksgiving and Christmas.
-If your store is in a busy area where people are more likely to want a pre-made meal, you can have a deli, sushi corner, or some other specialized in-house meal prep.
-Make deals with fast food restaurants so they have a corner of the grocery store like McDonald's (of course you make up your own name for the fast food chain, and you can describe how big that chain is in the details)