My five simple rules for shopping at Amazon.

1. Avoid buying plumbing parts. Plumbing parts are far more complicated, with details that are often missing or obscure, ensuring that many people will order the wrong part even though it looks like the right part. It might be more expensive, but it is faster and simpler when you go into a plumbing parts store and tell them what your problem is, as they'll not only know what you're looking for but will give you advice about the DIY job you're about to do. Instead, shop on Amazon for the part that you think you want to get, and then ask the plumbing parts store person if that part will indeed work.
2. Avoid buying things shipped from China.  I have found that the quality of construction is a hit or miss, as well as the authenticity of the item. Plus, these things take three or four weeks to arrive, having to go through Customs, and, when you inevitably have to resolve a problem, it'll take several days because of the time difference.
3. Use thetracktor.com to check the price history on your item using the ASIN number, which is Amazon's own ID for each item that is sold at their website. Track the price and buy it at its lowest, not at its highest level. If you use thetracktor, you can set the price level and have them send you an email alert, or you can add it to your "saved for later" cart and Amazon identifies the price changes, every time you check your cart. One requires no effort except to sign up, while the other requires you to be proactive.
4. Read the 1-star comments. The poor ratings are usually split between those who are completely clueless and those with actual experience and greater knowledge about the item you're looking to purchase. Often under these comments, there are responses that are equally important, as they often include technical references to guide you to what you really want.
5. Pay attention to the seller ratings. Never buy from someone who has below 4.0 stars. If you think about it, few people leave poor seller ratings, especially if the seller resolved issues; they would rather forgo leaving a rating than to leave a bad rating. This bias can hide the fact that a seller might be dealing with a lot of fake goods or otherwise has a high rate of returns / refund requests. As a result, I believe that an "average" rating is closer to 4.0 stars than 3.0 stars.