A clear title to the property, verified by a realtor and conducted through a title search.
A proper boundary and property line, verified by a survey and property records.
Clear tax history, with all taxes paid, verified by a records search.
A property inspection (for termites, faulty wiring, etc) done by a certified home inspector.
Knowing what the fair market value through "comps" (comparable nearby property sales)
Protection against any legal action that the seller might consider in the future.
Making sure all the documents are properly drawn up and filed in a timely manner.
Those are some of the things that a professional, licensed, bonded realtor who is a member of a professional realty board in good standing can assure you of.
I'm not a realtor, but these are just some of the things we've learned when purchasing a house, using a professional realtor.
These are just my unprofessional opinions. I know that by not using a realtor, you can both save some money on a commission payment to the realtor. But it's kind of like being in a minor car accident, where the other guy says "hey, let's settle this privately. Here's $500 for your bumper because we don't want our insurance rates to go up, right?", because it just looks like a dented bumper. You take the $500, he drives off without any ID being exchanged, and then find out that your axle is bent, and your neck and arm start to hurt. At the hospital, they find that your arm is broken, and your neck has whiplash and you're going to need physical therapy. And there's thousands of dollars in repairs that your car is going to need, and you resort to putting homemade signs up on telephone poles that ask if anyone witnessed the accident, and it's all a huge pain.