
It is difficult to place a value on the watch as the selling price on eBay is generally unpredictable.
The one with the 'Buy It Now' of £900 that you refer to has been listed on eBay for quite a few months without any takers.
If you decide to sell I would recommend that you take some decent photographs and add a nice honest description, start the auction low to attract the bidders, sit back and cross your fingers.
If you have a minimum price in mind you could protect it with a reserve or you could try a 'Buy It Now or Best Offer' style auction and wait and see what happens.
The value of the watch is going to depend largely on it's non-working condition, if a prospective buyer believes that there is a reasonable chance the watch could be repaired or that it has parts which are salvageable they would be more likely to make a bid.
If I was forced (nuts in a vice) to make a estimate on price I would estimate based on the rarity and desirability it to be in the region of £90 - £130.