In the recent December issue of Vintage Guitar Magazine, eight vintage guitar experts were asked what condition issues damage a vintage guitar's value the most. Here's a summary of their educated opinions:
1. Good refinishing decreases vintage value by 50%; poor refinishing decreases by more;
2. Cracks, breaks, router work, or an obscured serial number can cut value up to 90%;
3. Changed pickups can decrease value by up to 50%, sometimes more;
4. Changed parts like a bridge, tuners, pots, caps devalue by at least
the replacement cost, if correct parts are known to be available;
5. A non-working truss rod can kill a deal, devalue by 50% or more, or devalue by at least the repair cost if a buyer's willing to take a chance.
6. A headstock or heel repair done well takes away 50% of the value.
7. On super rare, high-value guitars, a small detail like a new screw in the pickguard could knock 25% off the selling price.
New strap locks, re-fretting, swapping a plastic bridge on an acoustic guitar for a wood bridge, neck re-sets, bridge re-glue, these may not devalue a vintage guitar or may devalue it only slightly.
Hope this is helpful.