EVE Online has a very robust market system. There are user-set buy prices and sell prices for every possible item. The UI in SWTOR is a far cry from being able to handle a GTN set up like this, but it truly makes it possible for someone to play the market, if they choose. If you're not familiar with EVE, you can also relate it to the stock market. If you want to buy something, you have the choice of either filling a Sell order (buying at the seller's listed price) or creating a Buy order and wait for a seller to fill your order at the price you set.