The question is whether the auction activity constitutes a small business or a hobby--a distinction that's not always clear. For instance, is the person whose flea market finds fetch top dollar online operating a business?
The IRS offers a few guidelines. If the taxpayer depends on the income, acts in a businesslike manner and puts enough effort into the activity to suggest a profit motive, then it's a business.
Still unsure? It may be best to consult a tax professional.