There are a lot of very good reasons why Flash banners are not commonly used on e-commerce sites.
A Flash banner may be interesting, but I think they are way too distracting on an e-commerce site if you use them on every page. Motion distracts users from the goal of an e-commerce site--which is to get them to view products and purchase them.
Flash banners can slow down the loading time for people with slow connections. Many e-commerce sites are already slow enough due to the number of product images on some pages. There are still a lot of Internet users in rural areas in the USA who do not have broadband Internet access. Cable and DSL are not available to them and satellite is expensive and the connection sucks.
Maybe I am biased against Flash. I put it on the "bells and whistles" category, which means it does not add any value to a user in an e-commerce site. If something does not add value, then it needs to be dumped.
I do not mean to offend talented designers, but I see a lot of sites with Flash that appeal to designers who love creativity, but are annoying to users. I usually find a Flash banner or Flash advertisement to be interesting for about 10 seconds, and then they start to annoy me.
osCMax looks to be a good choice for e-commerce. I looked at that one about a year ago. I personally prefer focused support, rather than the fragmented mess that ocCommerce support has become. It looks like the osCMax developers actively participate in their support forum.
"It's inexcusable for scientists to torture animals; let them make their experiments on journalists and politicians." -Henrik Ibsen