It depends entirely on the item (and how it is manufactured) and the number of units you plan to sell. There is quite clearly a difference between selling 100

It depends entirely on the item (and how it is manufactured) and the number of units you plan to sell. There is quite clearly a difference between selling 100 items that are designed to be simple enough for you to manufacture in your garage, to selling 10,000 items which require injection moulding, custom PCBs long assembly steps, finishing, QC/testing, certification etc, are clearly two completely different challenges. But the first step is to build a prototype that people want, and then think about how you can make it easy for yourself to assemble (because chances are that will also make it easier for someone else to assemble).