Hi,
I suppose it comes down to affordability for people - buying genuine Jaguar parts from a dealer will cost you a fortune. Yes, these cars are prestige models by and large, and when new are bought by people who have money to spend. But for the second hand market, price will likely be an issue, as it seems to be for this member - and I'd add that my own experience with dealers has been less than great over the years. So, if you buy from eBay, you may get something substandard, perhaps, if you are unlucky. But they also sell good brands. I just got a Bosch air filter for my X type from there. I could have paid double for one from Jaguar. I shall change it at the right time as per usual and doubt that there will be much difference. It is buyer beware - and check for quality. Look at the reviews and feedback for sellers and that tells you a lot.