I've noticed an increase of similar listings on eBay of this kind as well, selling DA2013's and Naga12's for 20USD inclusive of shipping. As someone who purchases from China quite frequently, I'm 99% sure its' a scam or a fake as for someone to ship something inclusive of shipping / eBay / Paypal feels, the mouse itself mouse have costed $7-8 USD to make any sort of profit (which would still be quite small by any means). To my knowledge, there aren't any at this price point in China at the moment, that is real of course.
The listing has been listed multiple times by multiple accounts using the exact same listing format by two previous eBay sellers, very likely to be the same person. One of the accounts, after receiving a lot of negative feedback in a span of 2 weeks, has been deactivated. The other - http://www.ebay.com/itm/Razer-DeathAdder-2013-6400dpi-4G-Optical-Gaming-Wired-USB-Mouse-/111237322285?pt=Mice&hash=item19e642d62d
, has gone into a similar situation, receiving massive amounts of negative feedback, and proceeded to set his feedback to private. This third seller appears to use the exact same listing as well.
As to other sellers who sell at 35 / 40USD+, most of them are pretty legit in my opinion, and makes sense at the price point they're selling. Usually when a listing seems too good to be true, you know there's something up.