Scams include fake paperwork, phony buyer deposits (scammer sends too much in a bad check), purchases through a third-party (a fake attorney, etc.) and asking for personal information.
Without a professional, there is no one to call when things go wrong. Pre-approving buyers and checking them before an open house becomes much more difficult when you don’t have an agent.
Since the owner is not familiar with these processes, they won’t be able to spot out a problem (like leaving valuables out) as easy as agents can.
For Sale By Owner properties also tend to attract the seasoned investor looking for a great deal. This can lead to the owner making poor choices and negotiated poorly.
Mistakes happen. If the owner makes these mistakes, then they have to pay for it. In most cases, agents will cover their own mistakes, since it only falls on them.