That's one of the things I like about Yahoo Japan 
(don't know if Yahoo US does it too, since I don't
use my account there). . . When you leave a 
feedback to someone, that person gets an e-mail. 
I'm sure there must be a way to turn off those 
mails, but I think they're great because:

1) I know when I leave a feedback, it's also a 
way of reminding the seller to leave one for me.

2) If there's a problem contacting a seller, I 
can try to contact them with a neutral feedback 
and I don't have to wonder: "Will they see it?"

3) If I forget to leave a feedback, I'm reminded 
when I get the e-mail with the seller's feedback!

Many Sharp Smiles,
--Drac