Yes, you can just make a shop, but you are more limited by what you can sell in Small Businesses because you can only sell things that fit on Retail Surface slots. You can't sell anything that only goes on the floor, like furniture. GTW Retail still has it's place for a lot of shop types.
Cash registers aren't used in this form of shop, though you could place one as décor if you wish. Sims simply swipe the item and you get paid immediately. The upside is that there's a lot less interaction involved, other than maybe using Welcome to Business to greet new customers, and maybe using Sway to Buy to get sims to buy something specific. You can even just open the shop and let sims run on autonomy sometimes.