Authority for county to prohibit sale of property if on upset sale list

[Dave Hyer]

Hello,

I was wondering if anyone could help with terminology and authority of county (Montco PA) to take property due to outstanding real estate taxes. I was looking at purchasing a home from an owner, who was open to working a deal...but after doing some due diligence, I learned it was on the upset sale list. I was told that purchasing from owner is not an option because it was on the list, but have been unable to find a specific statue or ruling (??) outlining timeframe and other perimeters of its status. If the owner pays the back taxes it is off the list (Stayed), but what is the status at that point. Can he immediately sell. Could a purchaser pay the taxes through the owner in some arrangement? 

Any direction or input would be greatly appreciated. 

Thanks

Dave