It's Amazon selling the Fox shocks. Not Amazon sellers. Amazon is the biggest retailer in the world by far.
Rockauto- I believe, is just an aggregated site of independant parts stores. That's why one shock listed will have the "ASH numbers" and the other will be the "1BlZ numbers" stufff. Someone correct me if I'm wrong here.
Summit is typically only known by those who are into drag racing- it's tiny compared to the other two.
So of the three, if I had to guess, I'd say Amazon is getting he best price from the wholesaler- but it's likely exactly the same for everyone.