Reading Alliance chat tonight and seeing one of the more experienced members, once again, saying
"yeah...as I said, CCP should finally implement an alliance marketplace"
I can't help but, again, nod my head in agreement, we really do need something alliance specific for the marketplace. Being able to sell items to your alliance without the need for contracts would really be useful and is the next logical step to setting up private regional markets based.
No longer would everyone be able to see and buy items at one price but it would allow players to set prices based on public purchase and another for the alliance.
Integration into the existing marketplace UI would be the best way to go, there should always be only one marketplace. Just change the appearance and how data is displayed to differentiate between different sales types as a result no new UI will be needed.
Order scope could be shown using a new variation of the "Show own orders" option that the market currently has to show Alliance only scoped items in a different colour. This would allow people to quickly and easily see Public and Alliance orders, decide which is better and buy that item.