I personally don't purchase any kind of formulated products off of ebay (ex: makeup, skincare, food,) because I don't trust that they haven't been tampered with.
However, I did win an auction about 6 months ago for a set of 15 brushes for a little under $100, and the seller scammed me out of my cash. Thankfully I purchased through Pay Pal, and they got me about $80 back, but I'm still out $20.
Nonetheless, I still wouldn't buy product. I just don't trust faceless sellers. At least on here, you've got some history and can base your trust on their inputs on the forum.