I totally overlooked the word "reliable" in the survey question. I just answered where I bought scuba gear.The premise the survey -- measuring if any of the secondary marketplaces is a source of reliable used gear -- is invalid. That's a very different question that the title of the thread, which doesn't have the word "reliable".
Some sellers may do a meticulous job of caring for their gear, but it was unreliable from the manufacturer -- does that say anything about either the seller or the forum (craiglist, FB) where it was sold?
Some sellers do a better job of caring for or refurbishing used gear than others, leading to a buyer's perception of the reliability of the equipment, but that says little about the category (ie., LDS, or eBay, or Scubaboard classifieds) as a whole.
In addition to what you pointed out about the ambiguity of the term "reliable," it could also refer to the seller's reliability. As in, can you rely on the seller to deliver the item you believed you were getting?