It's like doublespeak from 1984. Pre-owned is a fancy way of saying that it's used. Someone else owned this before you, and then didn't want it anymore, so they gave it to someone else, and now this person wants to give it to you without making you think that it might be a little bit 'used'.

This phrase is most commonly employed when it comes to commercial purposes. They really want to sell you used stuff because they make more money. Profit margin on a used item is usually much higher than it is for a new item, because it costs less to acquire used merchandise. The only problem is that saying that the item is 'used' will discourage some people from laying out the cash. So, they say this little made up phrase to make a sale.

Making pre-owned sales is doubleplusgood.