I'm stressing is the fineness of the line between what you have rightly described as what is allowed and what is not. If you're saying that selling isn't to be discussed so whaeer would you put buying? Gven that the OP wants to know how to buy, in any logic, I could have shut this down but contrary to common belief in some circles, that is not what moderation is about.
This thread will show up in a search for help on this issue and the more clear it becomes as the thread goes on, the less it will need to be repeated in future.