You should get a decent price for it, if you know the exact model and year of manufacture that can help as Billingham are constantly tweaking the design and some may want a newer one, some an older.
As tough as they are and although they look the part they are not camera bags that I'd imagine folks would use over any kind of long distance, they are quite bulky and heavy.
With that said I did make a roycraft rucksack frame for mine once, it has plenty of attachment points for doing so.
