Answer:
Nothing. It would be near impossible to get a cashier to give you a weird look for what you're buying.
Explanation:
Cashiers scan hundreds if not thousands of items every day. They do not care what items you're buying as long as they can get their job done quick and efficiently. Besides, everything they sell is stocked by the store which they are working at. They may have even stocked those items themselves. Giving a customer a weird look would only hurt sales and therefore hurt business, and eventually the cashier.
A cashier might ask a question for the sake of making small talk though. For instance, if you bought every single watermelon they had in stock, they would probably ask what you're doing that for just to make the day go by quicker. If you bought handcuffs, duct tape, a shovel, and a butchers knife, they might just assume that you are a butcher and keep bagging things. Any other thoughts behind that (like if the customer was a actually murderer, for instance), would just be for the sake of entertaining the cashier and nothing more, to again speed up the long work days that come along with minimum-wage jobs.
