Good lighting and a window you can get to, to be able to compare fabric in natural light. Notions. I'm a gadget junkie. I want to try the latest thing advertised in the magazines and at shows. I know it can't all be stocked, but have the tried and true things too, not just a few bobbins and needles.
Different brands of thread choices.
And last but not least: Don't be a machine snob or allow any of your employees to be one. If someone walks in the door, they sew. Period. It should not matter one whit to anyone what machine your customer uses.