It could also be that since the word Sunsun is not used its a knock-off of a knock-off LOL.
Or Sunsun has put out another line that is not labeled as theirs to boost sales or the e-bay seller just didn't put Sunsun in the ad. Which I think is the case since I did find a couple of the auctions listed the black ones as a Sunsun (although they do not show the black ones on their site)

.. I do know Panasonic used to sell televisions under several different names that were exactly the same as the higher models internally to allow them to compete in a lower price bracket. All they did was change a few of the outside cues and used different names on them, but all were still made by Panasonic.
I know the JBJ is a Sunsun, but goes for $50 more than the same size Sunsun and with a lower UV wattage to boot 5w vs 9w. Only things I have heard are a real difference is the inlet and outlet tubes seem to be made of a little sterner stuff but I don't think that makes a $50 difference.
As far as a knock off canister filter (as I have heard some people claim) I will say it is also common today for one company to make parts and pieces for others and use those same parts in a design of their own. There are a lot of reasons this happens, but it does not make said company a knock off of someone else because they use the parts. Sometimes Aqua Filters hires Bondo Manufacturing to make parts A, B, & C for their filter, but to keep cost down Aqua Filters does not make that part an exclusive to them but allows Bondo to use said part or sell said part to other companies if they sell said part to Aqua Filters at a reduced rate. Then Bondo either buys parts from a third company to assemble their own filter or makes the missing parts themselves building it around parts A, B, and C that they already build. So in cases like this it does not make the filter an Knock off and I think that in the case of the Sunsun that applies from all that I have been able to find. Some one in the pacific region or Asia maybe able to verify this more but from what I found Sunsun seems to have a pretty good rep over there.
All I have to say for the money I have found nothing better and in a lot of ways these canisters not only can take on the bigger companies but beat them. I have to say after a year of running these I have been very happy with their performance. They have held up well are dead quiet and do a wonderful job on my 75 and 55 tanks.
It will be interesting to see what others have to say once they get their hands on them.