Would it make any kind of sense to have a store or shop that can only be found in the ethereal plane? Do the rules as written not permit that? I've read the sections in the DMG and PHB about it, but it still seems a bit fuzzy.
My idea was to have my players stumble upon a shady purveyor of magic items who does his business in the ethereal plane to avoid the prying eyes of authorities.
Follow up question: would the astral plane make any more sense? To me, that seems less feasible, but you never know.
Update: The players were transported via a modified version of the Cubic Gate from the DMG. I imagine this merchant having the ability to shift to the ethereal innately (right now I envision him as some kind of fiend) and he carries an item or two with him when he travels there.