Moving this to the "Make the Web Faster" list, since this isn't related to the Page Speed extension.
Keynote has some performance indices for base page load time only (just HTML as opposed to full page load):
The HTTP Archive is also a good place to look, but this covers homepages only:
You can also set up your own benchmarks at Yottaa with arbitrary URLs:
Creating your own benchmark through WebPagetest/Catchpoint/Gomez/Keynote/Yottaa probably makes the most sense, since I'm not sure how much value a generic e-commerce benchmark will provide. What are you planning on using the data for? Usually if you want to do this kind of comparison it makes sense to customize it to the types of businesses you are trying to compare. For example, e-commerce sites selling high end modern furniture are going to have a different performance profile than sites selling generators. Once requires high resolution images, and the other does not. From the HTTP Archive we know that most of the bytes on a typical page come from images, so this can have a big impact on performance:
If you give us more information about what you are trying to do we can give you a more specific answer.