It's an SBS if the barrel is less than 18" or the overall length is less than 26".
If it has a shoulder stock, it's a SBS - transfer tax is $200.
If it only has a pistol grip, it's an AOW - transfer tax is $5.
You need to apply to the BATF for permission to make the purchase. If the seller is a dealer, he should be able to guide you through the necessary paperwork. You'll need a local Chief LEO (Sheriff, DA, etc.) to sign the form to indicate there's no known reason why you can't have it; some CLEOs simply refuse to sign (see
http://www.1934.org for lawsuit to end this problem). Paperwork also requires fingerprints and photos.
You must tell (i.e.: ask permission) the BATF if you move or take it out of state.
No legal difference between 10" and 14".
Be aware of state and local laws that may prohibit it, or certain versions thereof (NY permits pistol-grip-only but not shoulder-stock).
What are you looking to get? What state are you in? Who do you want to buy it from?