In Italy it is common to see, as you say, people who sell almost new gear for a very low price. But there is another way to buy quality gear at an incredibly cheap price. There are some small companies that work as subcontractors for the big names of the industry, like Mares, Cressi or Divesystem (common here less abroad). They probably have almost or no marketing and retailing costs, they sell almost at factory price
For example "Scubatec" for regulators (considered here comparable to Apeks) or "Bags Sport" for BCD and other equipment
Bags Sport Diving Products - GAV DIR30
This one, a "GAV" that is BCD in Italian (giubbotto ad assetto variabile). For 270 euros you get wing accessories and steel backplate (you can ask to replace it with aluminum, I think you would pay more)
I don't know if they deliver abroad or if it is convenient after paying the customs. Anyway the quality is the same or even higher of products at a double price. Also the second hand market is somewhat strange for me
I got a used Suunto Vyper for 70 euro not manufactured any more. The Suunto Zoop is about 186 euros new here