i wouldn't worry about big department stores like nordstrom's. they're authorized sellers of the designers, so rest assured - you can check with the designer to make sure. these stores also have good, 'no questions asked' return policies in case you don't like the bag.
Perhaps the original poster isn't familiar with the Nordstrom department stores. Nordstrom is a department store that has locations all over the USA, so you can be reassured that their website sells authentic products.