Verified Forum Third Party Developers are allowed to advertise (under strict conditions) their products, please see the links below for more information:
https://forums.flightsimulator.com/t/read-first-how-to-post-as-an-addon-creator/331382
https://forums.flightsimulator.com/faq#heading--advertising
This applies to products such as an add-on aircraft, peripherals (e.g. if Honeycomb had a forum account), or Roman Design selling plans/guides.
Anyone can list freeware, but payware is stricter and can only be advertised by Verified Devs as per the link in this message.
The selling of new or used items by consumers in a marketplace manner (e.g. eBay or buy/sell classified ads) is not allowed.