There are no requirements by the state to test for radon and seller is not responsible for the test.
The inspectors I recommend always conduct a 'radon test'.
The properties that have come back with high radon levels, I've always worked to negotiate the seller pay for the $1,500 to $3,000 remediation system. The seller has always paid for it, however, this is not mandatory but a point of negotiation.
Find yourself a strong negotiator for an agent as well as a good inspector & lender! You'll make good decisions that way!
Good Luck with your move to north Idaho!
At YOUR service...