I know, you might not feel like giving it away for 5k. But unfortunatly that is the current market price for a second hand phone of that type. I had a K750 almost 3 years ago, I bought it at the end of 2005 for Rs 40,000 if I remember correctly!!
If the phone, battery and accessories are in really good condition with a large enough memory card (1 or 2GB) then 6.5k ~7k at most. I really don't think you can sell it for more than that....
What about the data/charger port?In mine the data cable/charger port wore off after some time; sometimes the charger and data cable didn’t connect properly unless I kept it at an angle.