Allmovie is mainly a database. A private version of the IMDb - and a very limited version in that they expect to be able to find all the information themselves whereas the IMDb threw it open to the world and so made the better use of the internet. Allmovie (AMG) has only recently started selling DVDs of some of the films in their database.
If you mention Amazon.com you should really also mention Amazon.co.uk, Amazon.ca, Amazon.de, Amazon.fr, Amazon.co.jp, Amazon.cn
As well as often offering better shipping rates (shipping from the States to anywhere is very expensive) the other sites also often offer titles that aren't available in the other country sites
You also missed Play.com
But rather than just compile a list of all the sites on the web, which will probably include quite a few disreputable ones, I think it'd be better to compile a list of recommended sites - along with the recommendations and any complaints about it. That way people could judge which ones to use.
Maybe also have a price comparison aspect to it. Which sites consistently offer titles at lower prices? Where do the different sites ship to (and where won't they ship to)? Do they do DVDs and videos or just one medium? Do they only sell DVDs in a single region coding? Which format videos do they sell?
If you put together a list that told people that sort of thing, that could be quite useful
Steve