I'm extremely guilty of vape accessory shopping on ebay when I'm bored in the office, I've had some awesome bargains off there and a lot of vendors are also selling on there, as mentioned Stealthvape, as well as Greyhaze, Signature Tips, Vapoursaints, Just-Vape and torchy the battery boy batteries are available on ebay too.
As said, just know what you're looking for and keep an eye for items that are crap, paying attention to the photos to see if they've shot it from a certain angle to hide something or deliberately aren't listing much information about it and being vague, seeing how many of those items have been sold and comparing it to they're feedback etc.
My favourite waste of money ebay item was a Private V2 telescopic clone, within 3 days the 510 connection had threaded rendering it useless and the over all finish of the device was kack, as it was my first mod when I received my nemesis (also off ebay) I almost pooped I was that chuffed with it haha. You learn from your mistakes anyway, I often cross reference different listings for the same or similar device to see which one looks most the most legitimate, information packed and trustworthy listing, and if I have any doubt's ill fire over a quick message to the seller.