OK it's silver(928), it's a London UK hallmark, date letter (u) is 1915/16 and the importers/distributors mark G.S is for one George Stockwell of London or Birmingham(Not sure running on memory here) who imported cases and movements from various Swiss makers. I should have mentioned the "swiss made" written on the movement would have given the game away to all but the most hard of seeing.
So Silver cased 1916 UK market "Trench watch" with 15 jewel Swiss movement. Anywhere from 100 euro(if you're lucky) to 300 depending on how the auction goes. Though IMHO 300 would be waaaay too much to pay for this. It's a very common case design, the only standout being the swinging lugs. Me? If I got it for say under 150 I'd be happy with that, but I'd be happier if I got it for 90.
Do u really think it will take hundreds of euros to get it serviced?Looks all original, including the crown which often gets replaced. The dial looks OK save for a few nibbles on the edge hidden by the bezel. The "red twelve" dials tend to get more interest(and more hype from sellers). The swinging lugs are less common, most come with fixed loops so that's a bonus. Brand is hard to discern. Many if not most are brandless. Branding was less a thing in the early days and many were local jeweler put togethers. It looks like a Swiss movement to me. A Schild or a fontainemelon maybe? 15 jewels so a step above many. Does the seller say what age it is? This design can be found as late as the 1930's, particularly in the UK market where it remained fashionable for longer. If it's silver it'll be hallmarked and that will date it precisely. Size is another aspect to consider. It should be at least 30mm if it's described as a mans watch. Anything under that and it's aimed at the ladies(though ebay sellers will often claim otherwise). 35mm(or rarely over) is the sweet spot and more valuable. Is it running? The regulator is all the way over so it will need a service, but so long as nothing is broken under a hundred euro should cover you(depending on the watchmakers prices of course. Michael Swift in the UK would be my go to man).
Value? How long is a piece of string? If you get it for 18 then that would be a "bargain", but I'd be surprised if you do unless it's badly described. I'd say a running example would go for closer to 100-200 euros.
I have a few of these early "trench" watches and have had more over the years. A good one is a nice watch to have and more robust than you might think, though avoid water like the plague. One of the most accurate watches I own is a 1916 Longines and I've worn it as a daily watch for months at a time.