Of course, sellers need more feedback. More feedback will make the sellers percent of positives be higher if the seller has a negative. Also, some buyers won't buy if the seller does not have several RECENT positive feedback.
Withholding feedback until the buyer leaves feedback is NOT THE WAY FOR A SELLER TO GET MORE FEEDBACK. Doing things that please buyers is the way for sellers to get more feedback. More buyers are pleased by a seller leaving feedback before the buyer receives the item.
When more sellers leave feedback, that causes more buyers to leave feedback for OTHER SELLERS. eBay's research shows this, and Psychology books also tell us that. Most people know intuitively that when you do something for someone, that person is more likely to do something for you.
A seller can get a little more feedback by asking for it, but I think most sellers agree that it is best if a seller does not ask for feedback. It seems to be all right if a seller say something like "feedback is appreciated" in the listing or on a slip of paper that has the item's name and date of purchase.
