As we have the newspaper in the game, why don't we use it not only for advertising the things we sell but also for posting orders for the items we'd like to buy? You could add to the newspaper another page with user's orders, and those we could fulfil in the same way as we load the train. Each player could be allowed to post one free order for a certain period, just like the free promotion in the shop, maybe additional orders for gems, like additional promotions, too. Of cause, the items ordered should be prepaid by the buyer, and the seller shold get the money as soon as he submits the goods.