After examining an item, I'm going to put it in my purse and leave the store.
That sounds very risky. Don't you think that you'll get caught?
No, the security guard never pays attention.
Stealing something from a store is illegal and you shouldn't do it. If you try to steal an item and the security guard does then you'll probably be banned from the store, and might even be charged with a crime. Even if you think you'll get away with it, you should still pay for it.
I'm only borrowing it.
Even if you fully intend to return the item to the store in the exact same condition, the authorities will not recognize your authority to take it. If you really have some kind of arrangement to borrow items, you need to bring this up with the manager, and not just try to sneak out, otherwise your actions are indistinguishable from shoplifting.