Where can I sell flat rocks?

A flea market would probably be your best bet.

Is there much of a market for flat rocks?

Stepping stones in a garden or as a sidewalk perhaps?

I would like some flat rocks. Though I’m not paying shipping.

maybe a landscaping business, it be cool if they had nice insides so they be also cut in half for awesome outdoor decorations.

Brooklyn Bridge.

Sorry; I chip the bluestone slabs from my creek. I bet that I could underprice you, however

Locate companies that etch sayings and symbols in rock—they usually use those smaller, rounded flat rocks because they can be displayed with the decorated face upwards, and are easy to transport. Or you might consider doing that decorative work yourself—it doesn’t take much in the way of tools, just an air compressor and sand blaster, and the adhesive material you cut the design into which blocks the sand as you scribe the rock.