Identify stone used for exterior veneer
Slate and shale are essentially the same thing, but slate is the "metamorphic" form, meaning it was shale, but changed into a DIFFERENT (more dense) form by eons of time and tons of pressure. So slate would not be fissile ("flaky") to the average expression of human force. 

That's shale, aka "flagstone". People use it for siding and walkways, but usually in larger pieces. It might have value to someone, but people generally want it to match the color of something else (more shale / flagstone, house or landscape structure paint, etc.) so I wouldn't waste money buying an ad to try to sell it.