I'm no tax expert, but let's say you buy a painting at a garage sale for $20 and sell it on eBay for $20,000, because it done by a famous artist. Well you have a $19,980 (less expenses) capital gain which is taxable.
On the other hand if you buy a CD by Lewdicrass and you hate it and sell it on eBay for half what you paid for it, you wouldn't have any tax liability since you're selling it at a loss. You may have a deduction against your gain on the painting.