I'd be quite concerned that if purchasing used you could end up with the knock-offs from Earphone DIY Labs. Have you guys figured out a way to avoid being defrauded in the used market? When you get these kits do they come in a Grado branded box?
There are actually multiple details to help us to distinguish, but as you said, yes we did might end up with knock-offs if the seller wanna fool us even we are careful already sometimes.
I can share some details to help you guys to find the differences (and it would be better to have at least one pair of original Grado to use as a comparison):
1. The most important: there's no box that comes with the knock-offs (but they could offer a box in the future if they want. This might be the easiest part to do).
2. The cables are easy to tell if they come with braided cables. Those are definitely knock-offs.
But if they come with original-looking cables, it can be much harder to tell. One clue is that Grado does have a certain style of soldering their cables onto the drivers. All Grados I have are highly consistent in the way of soldering. Also, sometimes cable cores attaching to the solder point of the knock-offs which you can see through the back grid are in different colors which Grado never used.
One more clue on cables: the original cables are usually glued to the wood cup (or metal cup), and the cable would be twisted if you turn the cup around. But the knock-offs depend on who made them. Some people may not be good at details and would leave the cable unfixed.
3. The color of the wood cups. If you are a professional in wood, you can tell the difference. All the wood cups sold on that website are not the same type of wood as the original models, respectively. In my experience, the GS3000e knock-offs are usually in a lighter color, such as red or yellow. Some of them even have paint or polish on the wood cup. The original GS3000e's are usually in a darker brown color (except for some limited versions).
4. The matte metal cups. Some knock-offs with metal cups would have "PS1000e" on them. But to all my knowledge, there're only PS1000s in matte metal cups.
5. The easiest way, listen and you'll find the sound is definitely different. Some of the drivers they use are highly identical in appearance, such as the "GS3000e" in the link. But the sound is more closely related to SR80e IMO. It's not a sound that a flagship should have. For other "models", it can be very easy to see the difference, for example, "PS1000e". As shown in this post, the unsealed driver is attached to a 3D-printed driver panel and glued on the cup. Also, the metal holder on the cup is rounded without edges.
6. Although Grado has many tiny differences in different time on different versions of their headphones, you can always check the pics from other original owners. If you found a never-seen matches of cups, slider, driver types, headband (color, padding, and alignment), then it could possibly be a knock-off. Because Grado fans are passionate about collecting and sharing different versions in their collection. If something rare appears, you can always find a post. Or unfortunately, knock-offs showed up much earlier than we realized.
Finally, good luck for all Grado fans, and you are welcome to ask if you are doubting your Grado is genunie or not.
If you are interested, I'll post some pics of the older versions DIY I made for myself which are left in my home city.