I've seen it both ways. Sometimes a great vendor will have a negative score, just because other competing vendors are trying to make them seem bad and schilling. I'd say a negative score is only relevant if they haven't sold much, that's usually a sign they aren't viewed as competition and are getting downvoted by legitmate customers that got screwed over.
If another user downvotes one of your comments or posts your score goes down. (only if the account has reached certain criteria)
same if with upvotes. The more the community likes your content. The higher your score.
/d/Dread/wiki?id=0f0519ab