As regards selling...
If you have a gold membership or above, you can sell your knives in the Exchange. You can also put a small link in your signature to your sale in the Exchange.
Other than that, commerce is not permitted in the discussion forums, or outside the Exchange. You are free to discuss sales with one another via DM.
ETA:
There was a time when, in another maker's sub-forum, there was a thread specifically to keep members apprised of knives for sale by that maker.
Unfortunately, over time, the system broke down as a result of folks selling without proper permission, engaging in chit-chat and off-topic conversations, and it became a nightmare for staff to address.
As a result, Spark decided that no commerce was to be engaged in outside of the Exchange, but members could put a small link in their own signatures referring to their own current sale in the Exchange. What he specifically doesn't want is folks making posts outside of the Exchange directing others to their own personal sales.