7 days

Are two of those days travel days? I would limit the cities so that you can stay at least 2 nights, but preferably 3 nights per city. I personally would pick only two of those cities., or three max. When are you planning to go? If you have a full 7 days (not including travel) you could do 3 nights in two cities like Rome and Venice and one overnight someplace more manageable in between, like Bologna.

What kind of purses are you looking for? The fakes from the street vendors or the real thing from the Louis Vuitton store???? As far as the trip if you have never been to Italy before I would say Venice, Rome and Florence...limited time in each, but 3 really different and wonderful cities. I found Milan to be better than I expected and really enjoyed it, but not in place of the others and Naples was so big and intense and while enjoyable not a top contender.
I did buy a fake Prada from a guy in Rome for about 30. And I went with a friend to the Vuitton store in Paris where she spent 1200...who do you think had buyers remorse. (I always equate what I buy to a plane ticket....her purse could have been a RT trip to Europe) what will give you the most pleasure. Also, lots of leather in Florence, of course.

Open jaw ticket into Rome and home from Milan or the reverse, Two days Milan including arrival day, 2 Florence and 3 Rome.

Susan: Have to throw my two cents in about restricting the number of places you can see in 7 days. You can search this site (or just scroll through a few pages looking at itinerary questions) to find many discussions about trying to do too much. Always worth remembering: every change of location eats up at least 1/2 a day. Packing, transportation, finding hotel, checking in, unpacking. Have you decided on when, and where you will fly into and out of? If the flights are not yet determined, do arrive in one city and leave from another as Frank suggests. Venice and Naples are too far out of the way for such a brief trip. Stick with Florence and Rome as primary, but a day in Milan, flying either in or out, will give you shopping opportunity in a great city with a magnificent duomo. Florence is the best for leather. Even in other towns you'll find Florentine products: I bought my purse in Montalcino. Here is a link to some excellent advice from Rick Steves about itinerary planning: http://www.ricksteves.com/plan/tips/itin.htm. On iTunes Rick also has a short planning video: http://www.youtube.com/watch?v=9wAQuliNkfI.

I agree with Frank. The leather in Florence in wonderful and you have a huge ion at the open market.
I would spend more time in Rome and Florence and less time in Venice, though wonderful. I didn't enjoy Naples at all but loved all the other cities. My suggestion: fly into Rome (2-3 days), train to Florence (2-3 days), Venice (2 days), fly out of Venice. Enjoy your trip!