Seals such as O rings are a major factor in this sort of equipment, and obviously need replacing from time to time. You need to make sure that all the maintenace has been carried out by the previous owner, who should have a planned maintenance log to show all if this.
However, the price of older gear should be alot less to reflect the amount it has been used. But again, everyone has a reason for getting rid of their old equipment.
You are more than likely going to hae to spend more over the long run to maintain a used, older compressor than a new one, which will come with some form of warranty from the manufacturer.