Selling jg m 16 will upgrade for free

Hi guys! Trying to sell an unused jg m-16a3. My friend has one just like it and it performs really well. I want to sell for about $120 but I will negotiate. I will even upgrade it for free (you buy the spring... $12) and I will guarantee my work in writing as I have upgraded and fixed quite a few gearboxes. The highest spring it can take is a m 120 right now without major gearbox upgrades which will give it aroung the 370-390 fps range. A new barrel would also be good as the tighter the barrel the higher the fps and more accurate it is. If your interested let me know and I'll send pics and anything else you need to know. Its a good way to buy a good gun for cheap!