I remember watching a program about venture capitalists and the internet bubble and some experienced VC talking about how many millions he had blown trying to get a scheme somewhat along these lines up and running. Maybe the increased prevalanced and usability of mobiles might make it more feasible, but it a massively non-trivial undertaking - certainly trying to do it from on top. Some sort of organically growing website that vendors/customers use might work, but it is one of those chicken/egg problems - unless you have customers coming to the site it is a waste of time for the vendors to have to deal with the trickle of orders from it, and until a large number of vendors are present customers will get little use from it.

One way I could see working is a widespread existing business collection/franchise spreading out into more and more products over time - so companies/organisations as diverse as interflora or starbucks might start by offering products in their core competancy online, maybe start allowing other companies to join the scheme to cover gaps in their coverage, and eventually spread out to cover a wide range of goods so the website/app grows beyond the original founder.