The general rule is if it is used even 1 month it's worth only half of it's original sale price. 'Flat screen' and 'flat panel' TV's are also two different things. Making this question harder to answer correctly. Flat screen TV's can be as much as 15 inches deep, while flat panel TV's are generally less than 3 inches deep. Since you said HD I'll assume it's flat panel, therefore $100- $150 depending on name brand. If it's a thicker 'flat screen' TV.... throw it away or recycle it. nobody wants them any more.