Pre-qualifying is best left to the expert loan officers whose career is based on helping buyers obtain loans. When you sell by owner, buyers are timid and afraid to ask questions. Buyers ask sellers questions all the time. The difference is that there are no agents in the middle, miscommunication of information and taking days or weeks relaying information back and forth. Buyers and sellers can't negotiate for themselves, only agents have that ability.
People negotiate every day, with their spouse, kids, and coworkers.
Negotiation on the home is a much simpler process when it's done directly between buyer and seller. Agents are not mortgage professionals. For Sale By Owner's participating lenders can provide the answers for both you and your buyer. People who sell by owner expose themselves to lawsuits. This myth is spread to intimidate people who sell by owner.