It depends. (Doesn't it always?)
It depends on:
- what your cash situation looks like,
- what your payment terms will be with the manufacturer of your inventory,
- what the lead time will be with the manufacturer once an order is placed,
- what your advertising lead time will be,
- what your experience level is with running a business and creating web sites,
- and so on.
If you want to go into more detail about those things I would be happy to elaborate. I am typically partial to Web site first and inventory later.