You have to watch the postage, and a lot on Ebay are delivered from the US & Hong Kong, so add on customs at 25%-30%.
Most of the UK based Ebay sellers, will be using the same suppliers we do, but as with any store, and as I pointed out above, some are cheaper, some are dearer.
You don't do your shopping in Asda, and then drive 5 miles to get the Mango's from Tesco as they are 30p cheaper do you? (Well I do )