"Spray paint coffee beans and use them as vase filler...much cheaper than those marbles!"
Sadly, that's not what this is a photo of. This is an actual product, but it's not designed as a vase filler. They are called Coffee Joulies, and are designed to maintain the perfect temperature of coffee. I know this because I am a total coffee addict, and have been dying to get a set myself (but sadly I don't own any yet, so I can't report how well they work). They are made from recycled stainless steel and are, in fact, quite a good size - not tiny like real coffee beans.
But as the pin suggests: can you spray paint regular coffee beans to get a cool metallic vase filler? Eh, maybe. As I've reported before on our lace light bulb post, metallic spray paint never comes out looking like real metal, so if you're after slick metallic stainless steel looking coffee beans, keep looking. Spray paint won't get you that finish. Another issue that I can imagine coming up is what makes coffee so delightful in the first place: oil. Most whole beans will have a wonderful sheen on them from the oils in the bean, making your paint unwilling to stick well to it. Not to mention that it would be quite hard to get beans--tiny little coffee beans--coated evenly enough that they would look nice. All in all, if what you're going for is something like the original image from pinterest, this is just something I don't see working out as you might want it to.