I've been thinking of building a "shopping mall" site using tradedoubler and CJ affiliates to drive the majority of the business. My SEO skills will mean it will get good traffic through Google (I hope).
The questions are:-
1) Will the merchants have any problem with being in with their competitors?
I have build a few shopping mall type sites myself in the past and yes they do make money. I have found sites that are more specific and only focus on a couple of merchants make more and are less work in that you spend half your time weeding out non performing and deactivated merchants.
I don't think the horse has bolted but shopping mall sites are a dime a dozen and most use the same merchants.