I've added a tag called "Trade".
Feel free make a thread tagged with that
It is hidden from "All Discussions" however if it's solicited (other user requesting information about a device) feel free to reference the listing.
There is a feature to direct entire threads directly to other users (recipients). No one else can see these messages unless flagged by one of the members of that discussion.
Confidential, or references to financial / confidential things (such as emails, invoices or where to send payment) should use that and not be on public space.