For me private messages mean exactly that. However, coming from an it background all the messages will in a MySQL database clear text any good admin could read them if wanted.
You do need to make it clear though if messages are being read as would royal mail If they did such thing
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For me this forum is an amazing resource lets try and all get along.
with all that.
One thing I would add is that the rules are not prominent or explicit. If there is something about personal messages, I have not seen it. That's either the confidentiality, or the protocol of how commercial suppliers are expected to behave in responding to 'want' postings.
Confidentiality is a tricky one, since anyone with access to the raw data can read the messages. That's data centre people as well as anyone with admin rights within the application. Anything else needs encryption and we're in a whole new field of complexity. The simplest way to administer messages is to assume they are no more confidential than emails, analogous to postcards. In the normal course of events, they are not public, but the postie can read them as can anyone at the sorting office. If someone is accused of spamming, or bullying by pm for example they should expect to have that investigated.
As to protocol, there are plenty of examples of non-commercial posted responses of the type'I have a couple spare, pm sent'. If that is not permitted or it is but the rules are different for a commercial producer it's not written anywhere I have seen it. Posting ads is there, but nothing about pms. Should the rules be different? Perhaps they should since otherwise far larger outfits who will always have stocks can immediately reply to any 'wanted' post. On the other hand, it does avoid someone wasting time posting a response with a couple of suggestions when the original poster has already had half a dozen pms from the regular suppliers who have an account on the forum.
Is a pm only response permitted? It appears to be what is expected by a commercial supplier in response to a direct request. That is, send pms to known individuals asking about their stock. Again, not forbidden to respond to a general request anywhere I have read it, but is it against the spirit of no advertising? The danger is that all suppliers would be expected to pm to every wanted post. While not exactly spam, since it's solicited, it's not far off spam behaviour. What about something novel? If you are the only importer or maker of some unique bit of kit or branded item, do you stay silent or send a pm to be helpful?