The exception you're referring to is this one in Part 97.113 (3), which is part of a list of prohibited communications: (3) Communications in which the station

The exception you're referring to is this one in Part 97.113 (3), which is part of a list of prohibited communications: (3) Communications in which the station licensee or control operator has a pecuniary interest, including communications on behalf of an employer. Amateur operators may, however, notify other amateur operators of the availability for sale or trade of apparatus normally used in an amateur station, provided that such activity is not conducted on a regular basis. https://www.govinfo.gov/content/pkg/CFR-2002-title47-vol5/pdf/CFR-2002-title47-vol5-sec97-113.pdf https://www.govinfo.gov/content/pkg/CFR-2002-title47-vol5/pd... It appears that the exception is limitied to notifying other amateurs of equipment for sale or trade, not making the actual payment over the air. For example, if I got a one-time virtual credit card number and read it over the air to another amateur to pay for equipment, I don't believe that would fall under this exception, so it would be prohibited.