According to Dr. Mike Roizen from The Cleveland, Clinic Covid-19 is transmitted by one of three methods. First, is by droplets after someone sneezes or coughs. Second, is by re-circulated air in a building in which there isn't a system to sterilize the air. Third, he said it can be transmitted by food new studies are finding. He said if you have food cooked by someone else that you pick or have delivered you should reheat your food to 165 degrees to kill any potential virus.
Thus, I believe there is a minuscule chance that your clients will get sick by looking at homes as long as they don't touch anything. Also, don't bring clients who are sick or have been sick in the last 2 weeks.
For your listings here are some ideas. Make sure the home is sanitized before showings, offer hand sanitizer and shoe booties at the door and ask that buyers NOT touch any surface. Look but don't touch. I would also add that your sellers open every interior door in the house so that buyers don't' have to touch the door handles. Or possibly offer latex gloves or alcohol wipes. I would encourage your clients to sanitize the house after showings too for their protection.
What to do with your buyers? Don't ride in the same car with them. Keep at least 6 feet of distance between you and them. Third, I would bring alcohol wipes as you will need those after you use the lockbox and open and close the front door. Finally, tell them not to touch anything unless they are wearing protective gloves or ask them to bring alcohol wipes with them too.