Ecommerce on DotNetNuke still seems to be a bit of an elusive target.  There are many shopping cart modules available including the widely acclaimed ASPDotNetStoreFront platform.  We are currently implementing a store with the DNN Version.  Overall the experience has been pretty solid but we ran into a couple of hiccups with tax and some other issues.  The #1 issue from my perspective is that there isn't a one size fits all solution.  If you have a small simple store many of the mainline shopping carts are the proverbial sledge hammer when you only need a hammer.  If you are building a complex solution then you might be surprised at the ultimate price tag including setup, customization, etc.  Many organizations look to DNN for inexpensive solutions - the price can start to climb quite a bit.   My company has built a catalog module (see www.dnntoolsetmodules.com) which is specifically targeted at DNN users who just need to have a catalog on-line with out a shopping cart/payment system.