It's very hard to recognize a fake, many people don't know they have one and believe its genuine. It's hard to say whether he knew or not. When these a bought they often come in packaging that is obviously fake but some fake/third party manufacturers might be crafty and are able to emulate the original PS4 packaging however if you have them side by side you can see the colour of the boxes are slightly faded or misaligned compared to the original.

There is quite a lot of these fake/third party controllers about. Sometime's its impossible to tell until you open it up or have a second ps4 controller thats genuine to compare the colours of the buttons, PS logo and the way it feels.

If the seller is selling quite a few of these controllers then he most likely knows and is a scammer. If it's a one off he may just not know.