When you have 98-100% positive feedback and 25+ or so, and a punk with 1 positive (even if they have no negs) wants to buy from you, you have to consider the situation. If they are a stupid eBay newbie and don't read your auction correctly, or do something that causes them to have a bad experience, such as not being able to pay with PayPal, and then getting mad at you for only accepting it, etc.. You don't want to risk getting an "idiot" negative, especially from somebody who can just make a new account and start over.
When your long-term account gets a negative, you are stuck with it because you don't want to start over.
If somebody gets a negative on their first or second transaction, chances are they will make a fresh account.
I prefer to do business with people who have similar or greater feedback because that way we both have the same reputation to lose if we mess up, and both parties will be more likely to work things out with each other.
I have over 50+ 100% positive feedback on my personal eBay account. I don't want to risk messing that up to somebody who isn't risking anything on their part.