Trouble with ebay is it's a market-place, not a single seller. What's on offer varies from fake through to top-end surplus, even from the same supplier. Getting a refund in the event a lemon arrives can be problematic,
I don't know of any way of guaranteeing an ebay purchase will be fit for purpose. Could be a bargain or not, it's always a risk. Mostly ebay has worked out for me, and the few occasions I've come unstuck were due to pushing my luck – 'bargains' that were too cheap to be real. My purchases are always made with the possibility it might go sour in mind, and I take the rough with the smooth. No floods of tears if I get one wrong.
In practice, I prefer to buy tools from mid-range reputable suppliers who sort problems out. RDGs costs are higher in part because the price covers a high level of customer service in the event there's a foul up. A fly-by-night abroad has fewer obligations, and might not give any satisfaction at all.
My ER collets came from Warco (metric) and ArcEuro(Imperial). They're all good.
Dave