Read what Phenom, Grphyon, and quikstrike wrote, they have explained in pretty basic terms what the tradeback system is for and what you did wrong.
People on ASC actually have LIVES (well, some of us do at least), Jobs, and responsibilities (Some of us have families). I doubt that selling something to you is #1 on their priority list. Also, learn how to check when the person was last online, thats a good way to see if they are just ignoring you or have actually been busy.
What you did, imo is the same as someone leaving a negative rating on ebay for slow shipping. Cause we all know it is hardly the sellers fault (unless seller forgets to ship the package) that the company that ships the product is slow.
If practice makes perfect, yet nobody is perfect... Why practice?