Weigh the watch, subtract 16-18g from the total weight to account for the movement and multiply by the $/g price for 9ct gold. Add about $150 to the gold value for the movement.
I’d suggest listing it locally. You might find a buyer willing to pay over the scrap gold value and you won’t have to deal with the headache of selling on eBay and paying eBay fees, which might bring your net sale price below the scrap gold value.