kulan123 is a different sort of scammer, although one I've heard about far too often. What this 'delightful' individual does is buy a gig on Fiverr, accepts the order has been completed and then leaves negative feedback without prior conversation with the seller. Now, why would you want to accept the order has been completed and then give it a negative rating?
The answer is simple; competition. By doing this, kulan123 would be able to bring down the sellers rating, and then kulan123 could launch his own gig in the same niche, get a friend to order it and then get his friend to give him a positive review and thus he'll be deemed as more reliable than the seller he scammed over.
Always beware of this, and if you receive negative reviews after completing the work and the buyer hasn't previously contacted you about them being unhappy with the work, contact the Fiverr admins who would be able to help!