Although a seller may have received very good feedback from many buyers, it's always worth looking at their most recent poor feedback. Things change - especially in the current recession, and businesses may well be struggling. So, what appeared to be an efficient and reliable buyer 6 months ago, may well not be so now.
It's particularly important to see how they deal with returns. Their policy may look good, eg, 12 months warranty, but in reality, you can return an item with a legitimate reason, and nevery see a replacement or a refund of your original payment.