I make cakes only as a hobby. I have read here for years and am now able to bake delicious cakes, great fillings. I can torte and level my cakes and for some reason I can make beautiful gumpaste flowers without a problem.
My cakes still look amateurish because even though I have Sugarshacks' DVD and read lots on here, my buttercream frosting is just not as smooth as I want it.
So, what I need is practice, practice, practice. Can I buy a cake dummy tier and frost it, scrape off the frosting and try it again or is the dummy only for a one time use?

TIA!

Nope! Not a one-time use! Use it over and over and over and over! When making the icing, leave out the vanilla and any other "taste" ingredients, since it's just practice and you're not going to be eating it.

I always just slapped the BC right onto the dummy. I never used press-n-seal, saran, or any other kind of treatment. When I wanted to change it, I just ran a spatula under the icing and it came right off. Run it under a little hot water, wipe dry with a towel, and start all over!

I find icing a dummy different than icing a real cake. I know people always suggest it but for me there is difference, so in my opinion while you are getting practice, you are also practicing on a different surface then what you would normally use when doing a cake.