Smaller diameter (~4") flexible ducts are used that can snake through the walls. 

@Pat L.So the drain pipe could snake inside the drywall to meet the 1/2 sink vent pipe (~8-10 ft distance) without issue? 

- NOCan Real Estate Seminars be better ran without the sleazy snake-oil salesman approach? 

We ran snake test on drainage and also tested washing machine - no dirty water coming issue discovered.

According to the plumber report, he "ran 25-30 feet of snake and saw lots of sludge build-up".     

He went out of his way to call a friend of his with an industrial snake to come and fix it for me.

Battery drills, sawzall, dremel, circular saw, bandsaw, Maxi-rooter sewer machine, pistol snake, propress, threader, feed through socket sets, open end gear wrenches, transfer pumps, etc.  

But also check throughout the entire house.I also recommend snaking the pipes with a camera.

Playing with snakes eventually results in getting bit. 

Use a snake to cut them, put root killer down drain, or, if none of those work because there's too much, then you dig down and replace the section of the pipe where they are getting in.