If a buyer isn't willing to work with your schedule, they are not serious about buying. Most agents don't pre-qualify buyers. In fact, quite often agents will show buyers homes that are way above what they can qualify for just to keep some activity going through the home so that the seller will renew their listing with the agent. 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.