Randy,
As mentioned in above post, if you are considering Radon monitoring, get training first from the NRPP (use one of its approved extension courses). Not "all" radon monitors meet current EPA, there are still monitors out there, CRM types, that do not take hourly readings and would not meet the minimum standards for real estate transactions. That being said virtually all new monitors do meet current EPA standards and protocols. Ebay does occasionally offer the older versions that do not. There are several types, and methods to radon testing and the protocols vary by type used. I would not get into marketing Radon until you have been thoroughly trained and tested; you are playing with lives here. Too many in our field fail to test appropriately and give customers, or sellers, failed results.
"Radon, Nature's way of getting even!"

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Abraham Lincoln