eBay's official recommendation on this is that sellers should not even think about opening a store until they have at least 25 or more listings on eBay at all times, but I would venture to go a bit further. Based on my 8 1/2 years of experience working at eBay and being an eBay Powerseller, I recommend that new sellers wait until they have been selling for at least 3-6 months on eBay as well, and until they are already generating at least $500 a month in sales.