Rob Armstrong may well take it in and sell it for you. I bought a marvellous mandocello from him a while ago by this method, and he charged £600. As regards value, so much depends on condition, type, tone, volume etc. As a guide, Rob will build you a new six string guitar without fancy inlays and stuff for around £1500. This is one of the best bargains around. I have two of his doublenecks as well, and I know how hard it must be to part with an Armstrong. Hopefully someone on here will hear your guitar calling to them. I certainly do, but it is drowned out by my wife calling 'don't you dare...'!
Good luck, and I hope it goes for loads of money. By the way, the album of people playing Rob's instruments was called 'The Master Craftsman', and a damn fine album it is too.
Tim