Cool idea. If you know how to scroll, I would draw several backgrounds and use teammcb's infinite scrolling project here: http://scratch.mit.edu/projects/teammcb/421895 to make the cart go forever.
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Simple. Create a shopping cart sprite that has enough costumes of its rolling wheels. Make it go through those costumes. Then simply have it glide from one end of the screen to the other, and then move back to point A. Put that in a forever script and you done.
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