A 'USP' is a 'unique selling point'. Something that differentiates your business from the millions of others out there.
A web business, like any other, needs a business plan even if it's only two sides of paper.
You can't challenge ASDA in on-line food sales, but 'www.worldhoney.com' might be a winner if you can tell your customers how and why a particular honey scores against all others.