**POOF**
I HAVE BEEN SUMMONED....STATE YOUR DESIRE
The Jackson King V double-neck is both expensive as hell and rare as hell. It had to be special ordered from Jackson as it wasn't part of their stock catalog. If it's a custom one-of-a-kind model even better.
Reverb.com is the best place to sell gear like that- it's like an instrument-focused eBay, so you'll be presenting to the right crowd. If you can get any kind of documentation supporting your claim of Dave Mustainte playing it- photos, video footage, certificates, letters, signatures...it will increase the value significantly.
The downside is it's an oddball item- doublenecks are cool to look at but not very practical to play, and the fact this is a V-shaped doubleneck compounds the problem. This
may limit the buying crowd.
Offhand, without seeing it and assuming you have proof of your claims, you could be looking at $10000-$15000 at auction. The history could up the price another few thousand for the right buyer. Another option you may want to consider is contacting
Norman's Rare Guitars and see if they may be interested in giving you an offer on it. Norm sells high-end guitars to all the guitar heros and may know exactly what he's looking at. Norm also has a Reverb channel:
https://reverb.com/shop/normans-rare-guitars
Keep up updated and good luck!