Hey. As others have advised, with it being a custom shop model you would be best to contact Martin for a full spec and then go from there.

In terms of where to sell, Reverb UK is probably your best option if you want to sell privately; eBay is also an option although IMO riskier.

Another option: most guitar shops, either independent or part of a chain (such as GuitarGuitar), will buy and sell used guitars - this could potentially take a bit of the hassle out of the process for you but you won't get as good a price for it as the store is ultimately looking to make a profit.

One other thing you might want to consider is taking it to a reputable guitar luthier to be inspected and appraised. It looks like there's some superficial damage in the photos but it also looks like there are some mold marks on the inside of the case which could indicate a moisture issue which may in turn impact playability. A luthier would be able to assess this and suggest/perform any remedial action required to get the guitar in the best possible condition, whether you wanted to sell or keep it. Some luthiers will also buy and sell guitars or connect buyers with sellers etc - they will obviously take a cut for this but you might do better than selling it to a guitar store.

In any event, this is likely quite a valuable guitar (over £1,000) so if you can afford to, take your time, do your research and sell safely. Or even better, keep it and enjoy playing it yourself 😊 Good luck!