Look around the markings for the pi mark
(I'm on my phone so if you've put up more than the one picture then I have not looked at them)
You are looking for the Pi symbol that looks like a wonky gate similar to #, not the word Pi
The other permissible option is another UK standard such as HSE/AL/FW_ (but I would not expect it on that bottle
It does sound a little iffy that the seller says they have been using it at events (if that's how they justified it)
It may be true, it's the responsibility of the user / person filling to make sure it is valid. So if bottle checks were slack or a person deliberately evades checks and fills are unsupervised then people have the opportunity to fill an invalid bottle
International bottles are built the same, however if the manufacturer has the right markings it means they have gone to the expense of submitting bottles to the authorised bodies to be tested to destruction
Double check all over the bottle for markings and harras the seller. All uk paintball forums require bottles to be to uk standards and in date, so if sold via a forum and it's not uk legal they have broken the rules
If they were open and said it's not to uk standards but they have used it in the uk then they have been honestly dodgy