I think non-premium players should have the ability to at least view the second-hand bus market, so that they can know what's in there. They can decide whether they want one of them instantly instead of letting a premium owner inform everyone every time there's a new bus on sale first. The employees can get their favorite buses more easily.
Also, a bargain system would be great. Sometimes the price can be a little higher for the buyer. A bargain system can make the price tolerable for both buyer and seller, so that the seller can know the expected price from the buyer and sell the bus more easily.