Girls; how can I stop being oblivious?

In this case at the store, your behavior was fine whereas theirs was off. It's not your responsibility to make a move if they're stalking you.

If someone's smiling at you and maintaining a good amount of eye contact, you can be sure she probably is interested (in one of these situations). So go ahead and say 'hi'. That at least breaks the ice. You can also ask a question about some kind of grocery they're holding, like, "Hey, is that on sale this week?" Kind of corny but it could work towards making conversation at least.

Girls will usually show interest by smiling or eye contact, so intoduce yourself, make some small talk about something like what school they go to or job they have, hobbies etc. and see where it goes.hope that helps :)