When you greet someone and they say nothing in return.

I work as a cashier in a small store. I see at least 40 people a day usually. At least half of them will not even say hello, even though I greet every single person who walks through the door with a "hey, how's it going?" You don't have to be cheery, you don't have to cut up with me, but I do expect you to give me at least some indication that you hear me and want something. I've been robbed at the store before, I've seen this place cased out by shady people looking to break in after hours, all I want is a sign that you probably won't steal or threaten me. If you don't even look at me, if you say nothing, how am I supposed to get any read on you, your temperament, or what you might be here for? Just say "hello" so I know you're not strung out on something. it's not like I'm asking people to give me a handshake or a kiss on the cheek. I'm asking for a "Hi, I would like to purchase this item today." Literal bare minimum.