What should I know about radon levels at home? What levels are safe and do they affect resale value?
I gave a pretty detailed answer on radon mitigation systems in a related question.  Rather than repeat myself, I suggest that you read that and follow the links, particularly the EPA's guide on radon mitigation for home buyers.
Radon levels outdoors are pretty much always going to be zero.  It's only enclosed spaces you need to worry about.
As long as you can get a radon system installed without significantly destroying the home, and the radon levels aren't insanely high (100+ pCi/L), radon is not the end of the world.  Tell the seller to pay for a radon contractor, or have them knock a few thousand off the selling price and hire your own guy.  I would suggest getting a bid from a reputable radon contractor in the area so you know how much is reasonable for the sellers to concede.  Make sure you also factor in the price of someone doing a re-test on the radon levels!