I'm both a seller and a buyer on fiverr.com. As a buyer you have to be cautious because there are a lot of scammers on the internet, not just on this site. Most will pull the same scam, offer you an unbelievable deal with a few days completion. They then send you a job completed notice, but in the text say they have started. If you respond without first clicking the button to say you have a problem, you will have accepted that the order is completed. What you need to do it click "Reject Order" if you do not get what you ordered, or you're suspicious of the seller's promise to deliver when he or she says it is done. That way the seller either delivers or you get your money back. Also, if you stick with Level 1 sellers or above, avoid sellers that have sold nothing, and stay clear of sellers with a rating of less than 90% positive ratings, you will be OK.
As a seller, you have to be careful to always produce what you say you will, by the deadline you indicate it will be done. If you don't, you may have your account downgraded to a lower approval rate of suspended completely.
I read a lot of poor reviews sighting customer support communication as a problem. This is my one area of contention with fiverr.com, they're slow to respond sometimes, but do so eventually. Unfortunately, if you are trying to report a con artist abusing their service, this delay can give the cheaters days to victimize more buyers.
Fiverr.com is far better than most micro-job sites though. With one I can not even get them to fix my log in, which is never a problem with fiverr.com. I am told an email was sent with my new password, but it never arrives. Obviously I do not bother with that site, but I wish I could access my account to suspend the gigs. Others my beef is with the delays in payment for gigs I sold. So far all have paid, but whenever they get around to it. Fiverr.com has always paid me instantly once I did request funds be transferred. As well, all monies owed me were refunded money for gigs that I rejected or were not done on time immediately.
I only ranked the site 4 stars out of five only because they really need to improve their communication, and because every few days there seem to be technical glitches. None serious, but with the poor communication, glitches too are enough to cost a star in my opinion... But what I can not do is fault this company for scammers abusing their service. Crooks online are everywhere, and as long as they are suspended once found out the website owner has done all they can. Anything online is a case of. "Buyer Beware".