Chimney Liner for Flue Gas?

[Dar Fornelli]

Had our inspection on a 6-unit apartment here in Kansas City last week and the inspector flagged that the old brick chimney has no liner and flue gas from the gas furnace(s) and water heater vent through this chimney. I know this is a health concern with these gasses seeping through the brick as well as a possible deterioration issue. 

What have others experiences been with this? Is this a must fix? Maybe Fix? Don't worry about it? 

We asked for the seller to fix it and they got a quote for $7K and are willing to credit us this value (no way I'd pay anyone this much to do the work). We'll definitely take this credit but I'm just wondering if I need to try and pay someone a couple thousand to rent a lift and install a liner in the 3 story chimney or just take the credit and move on.