You shouldn't. I've bought rather a few used items from WP posts...and sold a few as well. Maybe I'm just "trained" by ebay and online retailer orders, but the assumption should be that the buyer pays before shipping. Maybe the payment may not *release* to the seller for a while though, depends on rules of the transaction (e.g. I think Paypal for non-eBay purchases sort of 'escrows' payment until at least verifiable tracking info is entered).
Even then, buyers do have redress. (As I found out with one attempted purchase here.) Paypal only guarantees they can get the buyer a refund on eBay sales. But if you use your credit card to fund the paypal purchase, you can pursue a charge reversal there. It might make Paypal vs. an unscrupulous seller be the one out the money...but bottom line is that the buyer should have zero excuses for not paying up front. They should do their due diligence on sellers with low post counts, ask for eBay or other equivalent nicknames to check feedback, etc. But as a seller don't be bashful about firmly and politely expecting payment up front. It's not unreasonable at all. (And if you don't like the 3% that Paypal takes, ask the seller to split it with you, or set your asking price accordingly... to me, I prefer the electronic "paper trail" over a check which I can never know if the buyer really sent or not, and as a buyer I'd never give out a CC number even if a seller had a way to process it.)
As to the original question - here and maybe the scubatoys used forum are the only good places I've found for underwater used purchases (the latter more for scuba gear directly not photo related). I also haunt and have scored on eBay but generally as a buyer I think I get better 'deals' at eBay. So I'd suggest posting classified here first, then after a reasonable amount of time if there's no takers, try eBay. Depending on where in the world you are there may also be more local scuba forums or underwater societies with listings you could try.
