When may amateur radio operators use their stations to notify other amateurs of the availability of equipment for sale or trade?

Amateur radio operators may use their stations to notify others about the availability of equipment for sale or trade primarily when the equipment is typically used in amateur operations and the activity does not occur regularly. This aligns with the regulations that allow for certain on-air communication regarding the sale of equipment, provided it is incidental and not the main focus of their amateur radio activities. Amateur radio is primarily meant for non-commercial communications, so operators must ensure that any sales activity remains secondary and occasional, rather than becoming a commercial venture.

In contrast, the other options introduce conditions that are not consistent with the rules governing amateur radio use. For instance, the asking price or the relationship of the seller to the equipment holder does not dictate whether the notification can be made; rather, it is the nature of the activity (i.e., being non-commercial and infrequent) that governs.