A second test sounds reasonable to me. Assuming the first test was performed properly the 2nd test should come up with similar results and just confirm there is a radon problem in the home. Good Luck!
I find Radiation Data to be a good company to work with for remediation. Most solutions range from $1200 to $1800, but some can be minimal (significant crack in the slab that is allowing radon into the home).
The link will provide you more info about radon
Homebuyers guide to radon
General info on radon
Map of NJ and radon
Even if the seller retests.....will you still seek remediation? The best way for you to make that deicsion is to learn a bit more about radon.
Somerset is nearby also, and there are some unusually high radon tests there also... I don;t know whyu or how.. but When there is a lot of rain, the water pushes down onthe ground adn the radon seeps up into the house. I would not close until the seller remediates the problem. I have not heard of anyoen charging over $1200.00 to install.. which is basically a hole in the slab with a pipe and an exhaust fan.
Have it remediated and retested before closing. Some of the "good" home inspectors have invested in computer units that test and give a reading immediately. I have the name of two that can come in and test for you.
On the other hand, I never heard of any Radon in Greater Edison area, although theoretically, it can be any where. Mostly I heard of Radon is Nothern Jersey.
I am wondering how many years the seller lived in that house? If they live there for many years without any problem, there is reason they do not believe there is Radon. On the other hand, if the test from professional, it should be taken care of.