YOU MUST BE APPROVED BY AN ADMINISTRATOR TO START A SHOP.
You may set any price you would like for your decks, but your customers MUST pay approximately the same price you set (so long as they accepted to pay that price to start with), but they ARE allowed to haggle in regards to the price. If it's not as good as they expected, they will ask for lower, and a couple may give you a tip. DO NOT ASK FOR TIPS.