Working with clay flues can be tricky. They tend to not do well with moisture and over time do crack. At that time, repair is not possible, and replacing the flues or relining the flues, are the only options. Do you know if the flues have been professionally inspected, and if so, did the inspector notice any cracking inside the flue, further down the chimney?
In our experience, cracks at the top of the flue usually mean cracks somewhere inside the flue as well. If that is the case, than you're looking at a risk for carbon monoxide poisoning and worsening conditions of your chimney.
We have worked with several home owners during the selling process and would urge caution with this chimney system.
Please feel free to tap into our vast storehouse of information by giving us a call, (320) 414-0191, or visiting our website, www.centralmnchimneysweep.com