I would love something like this and had mentioned it I think a couple years back. But the idea then as it is now is that there is no way to fully protect player shops without admin protection, and no way to get admin protection without changing their rules for a vanilla server and some significant work.
Your best bet is probably to create a storefront and a storage area for each player and work like some kind of consignment shop. But the bigger question is what would you accomplish that couldn't be done in chat and the trade post already at spawn, that would have players work unprotected? That's been my challenge too as I've been wondering for awhile because I'm all about player economies.
Regardless, I'm interested to see what you come up with and wish you good luck.