How do eBay sellers sell so many expensive things?

So I was on eBay looking at buying a new GoPro and I came across a seller who was selling over 500 a month. I was wondering how this person got a hold of all of them, he also sells phones. I know there are loads of sellers that do this but where do they get them all from? They also sell them a lot cheaper than the GoPro website (like £200 cheaper) and I was just wondering how? Where can I find where they get them from if i was thinking of selling large amounts of gadgets? They must be able to get a large quantity for cheap if they are selling them very cheap. Thanks!

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