START
If successful, add more items. If successful, add more stores. If successful, add more items. If successful, sell top shelf space at premium. Gather and analyze ALL data. Identify highest selling and most profitable items. Find manufacturers of those items. Seed those products directly from manufacturers. Create own brands. Sell those products under store/new brands. Use your foot/online traffic, reputation, edge, consumer behavior data to cut out the middle man in select products. Continue to offer sellers a superior marketplace to sell their goods. Experiment with your product placements, ads and optimize. Gather and analyze ALL data.
REPEAT
Nothing new here. For more info see - Walgreens, CVS, Walmart, Target, Whole foods, Safeway, Costco, et al.