I think that I would initially begin with listings on aquabid, the buy/sell sections of most related websites, and [Ebay Link Removed] After creating a good business model from that, Id consider hiring a web designer, a financial consult and I think that you could really begin to make a name for yourself like that. Just my late night opinion after a glass of pinot, hah! I would attempt to provide something that your competition cannot. Whether that be your product srlection/availability, shipping cost/speed, or just your level of quality/care/service. I know absolutely nothing about business and very little about growing plants, I just like to put my opinion out there from time to time while reading through the depths of info on here. Good luck in your venture, I think its a great idea. The more people supporting this hobby the better.
Apart from all that, I like the idea of selling to the LFS, and if you can figure a way to market presetup vases/ornamentals it would be a hit..(if you can figure out how to ship a setup vase that required little work/maintenance.