Have them pay for it and tell them you will reimburse them if there is a structural issue to the exterior sewer line. 

Credit score shouldn't be an issue.

This may be a case where you are trying to force the issue by wanting to complete a flip transaction where your market is telling you its not the right time.

However, if you're using a discount company that's allowing you as the owner to set the rent price, approve or deny apps, etc. then your issue could be one of many things. 

This means expect tax increases, expect HOI increases, expect tenant issues, PM issues, capex issues, everything in between.

However, for the seller to net the agreed price, the price has to go up in which case you can run into the issue of the property not appraising.I think the fundamental issue is this requires a more sophisticated buyer and seller (especially to have a graduated commission scheme which is common in commercial). 

Now though, it’s a real issue because it’s affecting my rental income.

Although we didn’t have a move in move out list at the start of the lease, we have all the communication from the beginning to prove that they reported every issue and we took care of it.

Older buildings can be an issue, we had to put some spray foam around the heat pipe entries. 

I will check with my city officials on this issue as they've likely had experience with VRBO and ABB