How can I paint walls properly without sanding due to possiblity of lead based paint?
If the walls are in good condition then why would you even have to anticipate sanding?

Prepare the surfaces by cleaning them well using a bucket of hot water and TSP (trisodium phosphate) cleaner. Make sure to wear protective rubber gloves. This will get years of dirt and gunk off the walls to make it a good place for new paint to adhere. The TSP will even help to dull glossy surfaces.

I have also found that the cleaner "409" will also help to dull especially glossy paint surfaces but they do also produce products especially for the purpose.