I don't think there is any safe way, including FB. I know plenty of folks who have been scammed that way, in various manners. I don't know that any way of purchasing is safe, but if you have found SAFE WAYS STOP POSTING THEM ON THE INTERNET PUBLICLY.
No offense intended, and I feel for those of you who are vaping over there, but perhaps a private message option of some type might be wisest (if STILL IMPERFECT) as far as giving out contact areas and persons. It's probably fine to post the question, but really, the answers shouldn't be POSTED on a very well known, high traffic internet site, ya'll are doing yourselves no favors, honestly. I really, really mean that.
Best of luck, and I really mean that too....
Anna