I FSBO'd the sale of my last house...but a) I had a few people who were interested and b) I knocked 6% off the asking price. But I have done many closings as an atty and I knew what needed to be done beyond just offering the house for sale (enforceable contract, escrowing earnest money, appraisals/home inspectors a lender would accept, etc.). I still had to hold the ultimate buyer's hand to refer her to mortgage brokers, etc...something selling agents have to do as part of their job. I had committed to purchase a home before I had the buyer for my old home. On the offer of the house I purchased I did not make the offer contingent on sale of my old home and dropped a relatively large earnest money deposit ($25K) in order to get the seller to accept my offer (multiple offers were in play).
Wouldn't ya know it, the sale of my old home fell thru at the last minute. I had 11 days to find another buyer, get them financing in place and close simultaneous with the house I was purchasing. It cost me a bit of money to make it happen but I pulled it off. Not making the purchase of the new home contingent on the sale of my old house was a calculated risk but under the circumstances I stood to lose $25K as I could not put together a bridge loan to close on the house I was purchasing as the proceeds of the sale of my old house ($150K) was critical to closing the purchase.
I guess thE bottom line is that if you are not experienced in buying/selling real estate a good realtor can be invaluable in getting top dollar and handling a lot of behind the scenes problems. What chapped my azz when shopping for a new home was many FSBO sellers want to pocket what they're saving in realtors' fees and not budge on the sale price. My situation was unique and I doubt many realtors could pull off what I had to do to move my house (possibly by some ethical restraints

) in a hurry. I have no doubt the selling price of my house would have been higher had I listed it with a realtor but it would have taken more time.
All in all it worked out fine for me...I netted pretty close to what I would have if I had listed it but it did require some extra work on my end. And the buyer got a good deal in the process, essentially getting 100% financing but with equity built in due to the sales price. I doubt that is the case with most FSBO's. YMMV
For most sellers, however, a good realtor isn't expensive. They're priceless.