1. Look at what it is and ask yourself is it cool?
2. Look for damage. If it is damaged I usually I pass it up or I will ask myself is it worth the time to fix it or will someone want it as is?
3. Does it just need a good cleaning? Most of the time this is the case and usually the reason it is for sale at a ridiculously low price.
The next time you are thinking of getting rid of something old for something new ask yourself the question. Do I just need to clean it?