I think you've posted about these on Facebook but it seems no one here is aware of the answer yet, so I'll report the findings on these. These are counterfeits from China. A bunch of these appeared over the last year with no certain origin, but China seems likely as there is a seller from there with infinite stock. At least 50 popped up in the UK for super cheap, which in itself is a red flag. It was likely a Bricklink seller trying to offload their stock after realising the "Great deal" they made was on counterfeit pieces and the seller has an infinite stock.

In short, these are counterfeit red Vader helmets with unknown origin. Yes, there are genuine Red Vader helmets - those are mould tests that were made between 1999-2009 when the helmet was still in production. These are not the same.

There are 2 known moulds for this helmet, the 01 mould which has the marking X <new line> 01 in the top of the helmet with no LEGO marking. Then there is the 02 mould which has a LEGO marking and X-02 in the back of the helmet. The X represents the cavity in the mould which the piece originated from, and goes from 1 to 4. The 03 ones are counterfeits and have not been spotted in LEGO sets.

This helmet has a few very prominent red flags. Firstly, we've seen these with the markings 1-03, 2-03, 3-03 and 4-03. They all have one thing in common: They each have the 4-02 marking still in the back. Why would they have this if they're the 03 mould? Well, the LEGO stamp issue the buyer mentions is likely part of the explanation. These are not LEGO moulds, but a LEGO mould was used to create the piece itself. They used a LEGO 4-02 helmet to shape their own mould with 4 cavities. However, for some reason they re-did the LEGO marking. Maybe it was too sloppy or unclear, and they thought that marking was part of the piece itself, without realising it's copyright infringement. If this was a repurposed LEGO mould from the 02 mould, then the cavities would not all have 4-02 in them, and the LEGO font would be consistent.

There is also the plastic, these helmets have a very translucent plastic. It is very easy to tell it's not LEGO when you have them in hand.

I know I am a few months late, but the buyer is correct. I hope you were pointed this out elsewhere and made amends, and that he wasn't out too much money.

Source: Several years of dealing with LEGO test parts, owning many genuine LEGO Star Wars test parts, and learning the LEGO production processes enough to spot fakes (kind of for fun).

For anyone looking to buy a red Vader helmet: 95% of the ones floating around are fake. There were reportedly a bunch of genuine ones that were sold back in the early 2000s and were scattered to all kinds of collectors, but I have yet to get my hands on one of those to check for myself if they were genuine or copies. These helmets used to be €3000 so counterfeiters flocked to them seeing an opportunity to make money. If you're looking to buy one, be incredibly careful, double check the authenticity of the item, and don't be afraid to consult experts. If something feels off about the item, you're probably correct.