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In the course of a thread, let's say someone mentions that they are trying to complete a nickel set. Is it okay to message them that you have some dates that you want to sell? If that is allowed, will it be better to direct them to the Selling sub-forum and list them there or can you deal privately with the person if they show interest?
If you both qualify (post count and membership time), a "PM me if you are interested" post is generally okay, but do not carry out negotiations in the topic.