On eBay, the sellers I have contacted have all had 500+ feedback and all 98% + positive feedback.
Well, I would never buy from a seller that had 98% feedback (especially if he has that many responses, it means he had to have that many negatives to get to 98%) unless it was for a real cheap item. Certainly not for a 1000 dollar item.
My criteria:
1. Has to have been around ebay a while.
2. If been around for years, small amount of feedback is not as important as having all good feedback (in fact, these people feedback is much more important to). It prolly just means they don't use ebay that much. On top of that, sometimes you get good deals this way cause less people want to take a chance on them (I got my last macbook for 200 dollars off plus some software plus a converter to from the macbook to my monitor and it was at the time a month old design. The person only had something like 50 feedback but she had been around for years and it appeared she just sold stuff she no longer needed).
3. If they have a lot of feedback, I want to see 99.6% good. And I will go see what the negatives say (There's a tool to just find the negatives for you so you can read them). You do get idiot buyers. But you can usually tell when the buyers an idiot, and when the seller has issues with stuff (if the negatives are consistant about the same thing, guy doesn't send what is described often or tends to be late on shipping, you can be sure he has a problem with that). Also, check how seller tends to respond to negatives. Is he crass/defensive/angry? Or does he sound professional in his responses?
4. Posting has to be written intelligently.
I am more willing to fudge on things for low cost items (shoot, I got a nano for dirt cheap because I broke my rules, I bought from a seller with only 10 feedback that had been around a month but something told me it was a good bet but I checked out what else he was selling and what he had sold plus I used a credit card so I could use its protections), but very picky when it comes to something like a computer.