has anyone used this for applying blush stick on checks? I don't use foundation makeup...

blendSMART is best for smoothing and softening makeup lines and color. You can use the universal foundation brush included with the starter set to blend cream blush. Apply your stick blush and turn on the blendSMART. Go side to side and up and down until you get your desired look. It's especially great if you happen to apply too much color with your blush stick. Just blend out the extra product with your blendSMART. Happy blending and we do have tips and tricks videos on our website.

where can I purchase this battery?

You can purchase the battery at any store

Do you sell the replacement brush

Of course they do. I believe you can just go to their website and choose the type of replacement brush.

What is the warranty?

Thank you for your inquiry into rotating tech, the easiest and more efficient way to achieve natural coverage. We stand behind our innovation and guarantee a 1 yr manufacturers warranty from all our authorized sellers - especially Amazon. We offer great tips and tricks tutorials for easy blending and hope to welcome you to the spinning revolution.