The whole concept of farming feedback seems flawed to me, because green numbers shouldn't mean anything by themselves. Its the quality and contents of the feedback itself.
But people do put a lot of weight on it. Feedback = trustability to a lot of people (though it obviously shouldn't). I've seen people offering their services here and calling themselves a 'trusted user' just because they did a few deals with the right people. It's like a badge of honour to a lot of people and I suppose it gives them the edge over someone who has zero feedback so that's why it's worth something. Some campaigns have even started offering higher payments if you've got it which is ludicrous and just encourages more abuse.
Because this is an issue in the first place, I'd love to see an option to leave people feedback without it being included in feedback score calculation. I'd far prefer an Ebay type feedback system, where before you buy something from someone, you quickly check their negatives to see if they ship things that "don't arrive", or arrive broken, etc. The issue of retaliatory feedback could also be addressed by having a comment section where a person could respond to their own feedback.
You can leave them neutral. An ebay-like system is far from perfect as well and wouldn't really work on here for numerous reasons.