It's a lottery the condition depending on storage, I have had 1930's pellets in excellent condition (normally card packets but sometimes tins) so oxidation varies hugely, with proximity to Oak accelerating the effect I have read
You need to ask for a close up of the pellets to be sure, and compare to online examples as per my Flikr and others as I have seen examples of the ''wrong'' pellet on occasion myself. I too have sold odd tins with all sorts in, but am careful to say that the contents are ''wrong'' as tins are used for all sorts and I have found allsorts in different tins. It could be 100% innocent and the seller not knowing of course.
Back to your question.....Eley's seem to stay in pretty good order I have found but non-genuine are a giveaway by the head ''pip'' if you look closely, and by how well they are made.
ATB, Ed
ATB, ED