Having said that, there's ways to mitigate the risk (e.g., get 3 or 4 quotes - not just one! 

However, if you test and find a high level; then it seems to me that mitigation would be appropriate. 

My opinion is that Radon is the same as any other item that you need to mitigate on a property. 

I don't anticipate 14% maintenance or 10% vacancy, but thinking worst case scenario always helps me evaluate the deal and helps mitigate any unforseen circumstances.

@Steve King Another reason I didn't see mentioned below is also leveraged risk mitigation in an event of an unexpected economic shift. 

If they break the lease by providing 60 days notice, then you are required to market the unit to mitigate your losses, but they are still on the hook for at least 60 days.

The most stringent screening process still can't guarantee you won't run into this again, but certainly can help you mitigate some of those risks (ie. be sure to include a nationwide criminal background check, eviction history, credit score and credit history breakdown, and reference checks from prior landlords).

We had a sewer scope performed on the property and there were no apparent problems with the line (no roots, damage, etc.)As far as the radon test, we didn't have to perform one since we later discovered the house already had a mitigation system in place -- so obviously it tested high in the past. 

Because you're not getting the assistance of an agent you will be responsible with some of that part and paperwork requests, you can mitigate that by having a clear and open communication line with your seller.

The deposit should be used to mitigate loss as per the local laws.