Everything But Walnuts is not a person but a store. They have... well, everything. Except walnuts. Or thing based on walnuts, like walnut oil or items made out of walnut wood.
But besides the walnuts they do carry pretty much everything. Weapons, secrets, vehicles, ancient kung fu manuals, out of print comic books, antiques, relics with mystical powers, you name it. If it can be found for sale anywhere, it can be found for sale here. The shopkeeper wears a nametag that says "Not For Sale" to remind people that she's not part of the store's merchandise. Nobody knows her name beyond that so they tend to just call her Not-For-Sale or The Shopkeeper. Some people call her things like Walnuts but they're no longer welcome in her store.
The shopkeeper reserves the right to refuse any sale and she makes frequent use of it. On the other hand, she also gives huge discounts on certain sales that would result in potent items finding their way into the hands of a worthy owner. Sometimes she'll even throw in a kung fu manual for free with the purchase of a packet of gum. Experts note that the store is subtly working to fulfill a lot of grand destinies in this fashion. As such, the place has the gratitude of a hundred heroes and villains. Only a moron would ever dream of robbing the Everything But Walnuts store.