You read about this or that being more expensive on the ship, i.e. drinks, suntan lotion, sandals, etc. I was wondering what you found is actually cheaper on the ship than on land. The only thing that comes to mind for me is:
With the bottles of liquor, know what you are buying and the price in your local supermarket. Over the years I have noted that "bargains" in the gift shop where liquor is cold can be purchased cheaper in our local supermarkets, and you do not have to lug the liquor home.
You can occasionally do well with a piece of art IF you know your stuff.
I collect animation art and have some idea about what various pieces are worth. A couple cruises ago I picked up a cel that I really wanted for hundreds less than I could sell it for tomorrow.
But I never venture into art purchases aboard ship in areas I'm not well versed in, unless it's an inexpensive piece that just gets under my skin. That's happened a couple times. In a case like that you just take a flyer because, good deal or not, you'll probably never see it again.