jnelson260,
First, Welcome to LTG! You may even find someone here on the forum who might be interested in your D-25. I am sure folks here would like to know more about it... '73 was a transition year for the D-25... form its original, mahogany top with a flat back of solid mahogany (pre-'73) to a spruce top (usually stained) with an arched back of laminated mahogany. During the transition, there were some mahogany-topped laminated arched back D-25s made, as well. So your D-25 could be any of 2 or 3 variations in build.
The difference between flat back and arched back is obvious... in addition, the flat back is braced inside, the arched back isn't. Mahogany vs spruce top often confuses folks... they see D-25M as the model name and think it means its all mahogany... usually that would be a "mahogany-stained" spruce top. Mahogany tops have a more prominent grain. The mahogany and spruce tops would be easy to distinguish by looking at the undrside of the top with a small inspection mirror (mahognay is a darker wood that would match the back and sides, spruce is lighter.
As for value... D-25s sell pretty commonly on eBay, with prices ranging from lows under $500 to highs over $750, depending on the condition. If the condition is as you describe, you might expect it to sell at the high end of this range... maybe even exceeding it. There are important factors to consider in addition to appearance... one being the angle and straightness of the neck... which would affect the action of the guitar. Buyers will probably want to know (1) that the neck is straight (and the truss rod works), (2) the action at the 12th fret... measured from the top of the fret to the bottom of high and low E strings, in 32nds (or better, 64ths) of an inch, and (3) the height of the (bone or plastic) saddle above the (rosewood) bridge.
Good luck! (I'd be very interested, if I hadn't spent my $$$ on a D-50 last fall).
Dave
