Many companies take their leftover supplements that have expired and put them on Amazon. Yes, there have been some third party sellers that will purchase the expired products, re-label them and then slap them up on Amazon.  They are also much more likely to sit on the tarmac (think extreme temperatures) for several hours, or in a cold warehouse for months at time. This inevitably has effects on its quality as it degrades the important structures. Also, none of these supplements are regulated so unless you are buying from the source, you have no idea what product you might actually be purchasing. Remember, the most expensive supplement you buy will be the one that does not work.