I like the idea, but I'm unsure... would this make the player shop NPCs even more redundant?
There's the Promoter NPCs in Vertick and then Shopkeepers in Housing. And the Marketplace NPCs which shows the lowest priced items first.
On the other hand, this would be interesting to use especially with the world in Revelius. I can imagine players doing long ranged trading through /shop without being near each other, going through a mailbox, or going to the marketplace.
If this idea was implemented though, of course It would be easy to do /shop [username], but I honestly would prefer something a bit more elegant such as holding a 3D item (like the newspaper or gold coin) and then clicking on it which opens a GUI that says to type the player's name (This GUI would be similar to the Mailbox system when you send something to a player and have to type their name first).