Group Buying Widget

Site is not up yet – still in the design phase and I am new to WP. Would like to add a feature on our site that would allow users with accounts to be able to purchase things in bulk quantities, therefore lowering the price. The difference is – I would like the customer to login, post the product that he is purchasing, have that post be visible to other members to see if they want to order the same item – lowering the cost for everyone. How deep of a rabbit hole am I jumping into here…