Has anyone here ever purchased any skincare products from eBay?

I've been trying to find the right set of Estée Lauder skincare products for my mom for her birthday (and for that, I'm definitely going to be buying through a department store/Estée Lauder's website), and my search sent me to eBay. I'm looking at products (hello, Idealist serums and such!) that I would love to have but can't afford at regular prices. I'm wondering how legit these products are. Does anyone have any experience with buying such things off eBay? Are they actually what they say they are? Have they been altered in any way? Are the eBay results a perhaps mixture of legit Estée Lauder products and crappy other fake/knockoff stuff? Anyone have any suggestions on how to determine which products to avoid?

I'd love to hear your tales or advice!