My grandfather gave me his Sigma 12 string a few years ago. I have a beautiful Martin guitar, and the Sigma has just been sitting in the case. Now, I'm interested in selling it, but I have no idea what it is worth. The stamp on the inside says DT12-4, it is a 12 string, the headstock says SIGMA est. 1970 in something that bears resemblance to mother of pearl. It has sunburst sides, back, and front. It is in excellent condition, though I did have to replace one of the original bridge pins when I got it. No dings or dents, and the neck still retains its shape. Anyone have any idea on what is approximate value might be?