Hello and thanks for trusting me to help you today. I am a tax adviser with over 15 years of experience.
I have to disagree with some the information given above, especially about the reporting of the sale on "line 2 on form 1040".
Let me explain, almost everything you own and use for personal or investment purposes is a capital asset. When a capital asset is sold, the difference between the basis (usually cost) in the asset and the amount it is sold for is a capital gain or a capital loss.
Capital gains and deductible capital losses are reported on Form 1040, Schedule D (PDF), Capital Gains and Losses, and on Form 8949 (PDF), Sales and other Dispositions of Capital Assets.
Gains are added to your income but losses on personal property are not allowed to subtract from your income because they are personal use property.
Now you stated "but I am not doing that all the time". If you are making money doing this on a regular basis and it is not an occasional sale, you would just report the transaction
as I stated above on the forms. When trying to make this choice look at it like a garage sale (happens maybe 2 times a year. Not an ongoing activity. If you are doing this more often you maybe in a business.
I sincerely XXXXX XXXXX information is useful.