When buying audio, I buy what I want and don't really consider resale value. However, almost everything I buy is rather high performance, so it tends to hold value relatively well. I tend to hold onto gear for a long time - sometimes decades even.
I disagree with tonykay that it's like the stock market. The major stock exchanges exist in large part to ensure liquidity; there is no equivalent in audio. Moreover, audio equipment is not an investment, it's an expense.
