Well if you are asking a general question, then I would say after you have enough inventory to show and sell the products at one time and second when you have enough and reliable feedbacks and good comments under your belt Obviously I will buy from a person who is a reliable seller and has decent feedbacks without issues.
Best time to open eBay store depends on the product you want to sell. So to start eBay store 1st thing is to do research on product you will sell on topics like when the products can be used more etc.. Other things which are necessary to start you store is create you paypal account, shipping methods...
According to me, the different tools available to the eBay store owners really help high volume sellers. For example, if u sell a large number of items on eBay, it will be difficult for u to handle listings, invoicing, payments, deliveries, and feedback. But, once u open your eBay store u get access to “Selling Manager†that makes all such tasks easier. This frees u from the mundane recurring tasks and allows u to spend more time developing and executing strategies to increase your sales and profits.
This is what I am thinking about. For my part, if I have enough goods and a decent profit, i will open a store on ebay.