Nikon D3000 SLR camera, equipped with Golden EC-300W flash unit, shoots shoes in the room. How should the camera be set up?

Actually, you don't need a flash to shoot still life.

My DIY soft box, plus two small energy-saving desk lamps, works well with the card.

What you said is irrelevant. The point is not the shoes.

Put the flash and shoes away, turn on the indoor fluorescent lamp, use the manual exposure mode, set the shutter to1100s, shoot, adjust the aperture until the exposure is accurate, use point focus in the focus mode, and shoot after focusing properly.

In fully automatic mode, the focus may not be on the shoes, or the light is not turned on, and the indoor light is too dim to focus.

As long as the SLR is focused accurately and exposed accurately, it will be very clear when used on Taobao, which is enough for you.

Feel for yourself. Good luck.