The depreciation is huge the minute you walk away from the retailer's doorway.
Better said, it's a devaluation. The psyche of the SCUBA shopper, at least 99.99% of people who are first-time-buyers, is that the gear must be new in box with the warranty card. These days, fewer and fewer people are stating this expensive sport.
When you are reselling gear in the used market, your audience of potential buyers is a mere fraction of the already small universe of new-gear-buyers as above. Again, most all will buy new, most of those will drop out fairly quickly and resell their gear on Craig's List (immediately or in 18 years). The used buyer type? Now you're dealing with habitual lifelong divers. A swarthy lot.
Looking at the majority of asking prices on Craig's List can give you the idea that somebody- either you or them- doesn't know what an out-of-hydro steel 72 with webbing straps is worth or maybe what that Healthways tunic top will fetch. I think it's the seller who has overestimated, but I could be wrong.
On eBay, I see a fair market for very recent models of regulators, the tech leaning BC's, tech gear in general. Computers and dive watches can do well.
There is no market for low-med end consumer fins or masks... not much on even high end masks, maybe it's the ick factor. Snorkels? Take a guess... that and most wetsuits- if they're not likely dry rotted, there is the lingering size issue and the inability to try it on, return it... and again- the ick factor because of the warhammer becoming common parlance what with the Googles and all.
I sell all sorts of things on e-auction sites. I write a good header- that's where the search engine hits- lots of photos and descriptions, i don't hurt people on shipping- and I start everything at 99¢ and select 10 day durations that end the auction at 7pm Pacific time. ebay (and auction sites) will tell you what something is really worth.