Purpose of Star Icon on Trading
If it is a gold/yellow star in a circle, it denotes the price being nearly double (or half) of its usual value. If the price is green, this is a good thing, and if it's red, you should probably avoid buying/selling at that price unless absolutely necessary.
The star is an indicator of high demand for that item - the trader will pay around twice the usual price for it (usually +90% to +110% on the selling screen).