Don't just look at the number. A seller may have a total feedback of over 5000, but may have several hundred negatives. Many of these might be retaliatory negs, but might also indicate that the seller does such a high volue that many buyers slip through the cracks. Some sellers have such a high rating that they don't care if they get a few negs now and then. Give them some slack, though.
Much negative feedback is retaliatory.
Only by experience can you sometimes, but not all the time, tell the difference. Also check the feedback that they have left. Some sellers just don't have time to leave feedback. I saw a seller once with over 1000 feedbacks, mostly positive, but less than 100 feedbacks left. I asked him about his feedback policy but never got an answer.