I like the part about unsold items being returned to you via the post office -- hadn't thought of that!
I think 7 days really isn't enough. If you don't want to go with 30 days (my preference) then how about 15 days? Split the difference kind of thing?
Also, Gramlath had some ideas for a quest tower, which could incorporate shops. I suggested that a quest would have to be completed (given by a town NPC) to gain access to this area, and successively higher levels of the tower (which would offer higher-level quests & shop items) would be accessible by completing higher-level quests, given by town NPCs). This sort of consignment shop could be set up in the tower ....
And, um, Lippy & Gramlath were talking about a ways to completely re-do the shop system to make it a bit more elegant and in-line with how other CRPGs do the buy/sell thing .... which might change how a consignment shop would be implemented in the game.
But still, if you can implement it sooner than the overall buy/sell redesign, that would be really cool!