There is already a 5 (or maybe 7?) day period for new eBayers. They can't leave feedback (neg, neut, or pos) for the first 5 days. I think this is a good idea as it stops feedback harassment. Someone can't create a bunch of new accounts, buy one Buy It Now auction from a seller using each of the new accounts, and then leave a negative feedback from each account. They may be able to do so after 5 days, but it definitely cuts down the "instant gratification" of these kinds of idiots. In all reality, they're not going to bother their feedback harassment plans if it's going to take them a week and a lot of effort to do it. Secondly, it stops scammer sellers. They make several new accounts and leave each other positive feedback for bogus auctions until they all have 10 feedback, the magic number to start selling using Buy It Now auctions, at which point they can make even more bogus accounts, make 1c Buy It Now auctions and get even more feedback, and finally sell fake Plasma TVs for $2000 and run away to Mexico. It definitely slows down that kind of shenanigans.
I don't agree with forcing new eBayers to rack up a rating of 10 before they can leave any feedback. What about someone who wants to buy something on eBay, so they sign up, and buy it. When the item comes, they try and leave feedback, but they can't and they will never be able to. This violates a fundamental right of eBay members which is the right to leave feedback in a publicly displayed location about those they have dealt with. You can't take away that right, according to eBay's rules.
I'm afraid that this just isn't the answer. I don't know what is.
Currently, if a new member signs up, buys something of mine, and leaves me negative feedback, I get the feedback removed on the grounds that they signed up specifically to harass me with negative feedback. It works.