before opening a store - Do you have any actual experience selling anything on eBay? if not get experience

Do you know anything about running a business?
Can you afford to pay for rent if the business goes under before the lease is up?
After researching the above - try selling for other from home first - get money up front for the ebay listing fees and at least a $1.00 per item for your labor to list the item on ebay - it takes time
learn how much to charge for shipping in advance and put that charge in the auction listing or if you are shipping UPS, the charge will depend on the destination from you location, so list in the ad to have them give you their zipcode for a shipping quote (you’ll need to be able to weigh the package and with the weight and zip, you can check the shipping costs on UPS.com or USPS.com
If you have a paypal acct - let the people you are selling for that if their listing sells and the seller says okay to accept Paypal payments, then the net amount they receive will have the ebay sales commission AND the Paypal fee deducted, PLUS also, whatever commission you decide on for selling and shipping it for them (I would say a minimum of 20% commission / at least $1.00, so in all - the seller might only get 60-70% of the selling price - if they think that’s too much of a charge - they can figure out how to do it all themselves - they will still have to pay the listing fees, ebay sales fee, and Paypal fee if they set up their own Paypal acct. The 20+% commission you charge is for your knowledge and labor