The way it was explained to me by the 20th IRS agent I spoke with is if you sell the domain as a brand/trademark/established business then it can be claimed as capitol gains, or in some cases as a casual sale ie you sell one name every few years and isnt a consistent business practice of buying and selling names, although they stated this is still a grey area because there are no court cases which ruled either way and being congress hasnt passed a law yet, the IRS sets policy and waits for case law to back them up or prove them wrong, They also said if you buy and sell names as a practice then domains are treated same as products and a schedule C would be used, domains being inventory. I am not an attorney just relaying what the IRS told me
Joe T