I've bought all sorts of stuff, off of Amazon and never had a problem...BUT having said that Amazon has changed in the last 3 to 5 years, they no longer give out good description information like they use to, and they no longer tell you if the seller is an authorized dealer for the product you want, so now you have to ask questions to get the information. If the seller is not an authorized dealer then you'll have problems with any warranty issue, and could run into a counterfeit item.
I've been buying bike parts for years, and in the last 25 years I've haven't bought anything from Jenson USA because they haven't been cheaper on anything that I ever needed. Now having said that it is wise to check several bike shops on line to see if they have something for less than Amazon, I've found a Serfas saddle for my wife cheaper at Serfas than I did on Amazon?! Strange, but it just goes to show you that you need to do some shopping, the biggest problems in pricing is usually the cost to ship, Amazon is most of the time free, but shipping from various stores can charge you from $5 to as much as $20 for the same item!
Ebay is a lot like gambling, you hope that the seller, who appears to have a lot of buyers with a high reputation score would be good to go, but China is smart, but crooked, they will load up glowing reports on a seller (they do this with reviews on Amazon too), because the seller and or the product is coming out of China, so they want you to think this product is good, they load as many as a 100 reviews, all 5 stars, and all fake. So I try to find a product with more than 100 reviews on Amazon; on Ebay I simply make up some question and ask the seller, if they don't respond I don't buy it from them.