The myth that Amazon “sold their excess computer capacity” has been disproven for years.
Just think about it. If Amazon had excess computer capacity and sold it to other companies, what was it going to do when it needed that excess computer capacity? Kick others off?
Yes, that's what preemptible spot pricing is; you pay a discounted rate for some compute time with the knowledge that you could be bumped out at any time.
Alternately, Amazon could lease time at an hourly rate which varies with available capacity; just raise the hourly rates around Christmas because you need them for your e-commerce site.
I know what spot pricing is. I work at AWS in Professional Services. AWS was always built from the ground up as a separate service. Amazon Retail is still migrating some of their infrastructure to AWS.
It would be slightly inconvenient if customers needed more capacity at the same time that Amazon Retail needed more capacity.
Just think about it. If Amazon had excess computer capacity and sold it to other companies, what was it going to do when it needed that excess computer capacity? Kick others off?