I think most computer sales people fail to truly appreciate the value of a consultative sales approach. So often, they take for granted that if someone is in the store, they MUST want to buy a computer.
However, by spending a little time getting to better understand your customers wants and needs, it can become very easy to establish a long-term customer.
Look for someone who has a good ear, and legitimately cares about the needs of your consumers, and you'll be sure to have made a wise decision.
Of course, it won't hurt if they also have awesome closing skills, and know how to promote the business in other ways to keep the door swinging open with new customers.