Amazon will dish out thousands of discounts during Prime Big Deal Days, its members-only "October Prime Day" sale (set for Oct. 8 and 9). But buyer beware: Not every "deal" is as good as it may seem.

Amazon doesn't have an unlimited supply of every item it stocks. And some of the more popular items could easily sell out during Prime Big Deal Days. If you have your eyes on a particular product, you may want to see if you can sign up for an invitation to buy it.

Go to the main Amazon Prime page and click on "Try Prime," then follow the instructions. Note that you need an Amazon account before you can sign up for Prime, but if you don't yet have one, it's easy to set up. Go to Amazon and enter your details to create an account, then add Prime to get the 30-day free trial.