Yes be prepared to lose about 40% of your watch price when selling to Wing Wah Watches depending on the brand and model though. I lost that much when I sold my IWC Portofino Chrono which I disposed of it after 1 month buying it brand new.
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By golly, that's quite a huge haircut for a 1-month old watch! BTA, these pre-owned shops need to take in the watches as low as possible to guarantee their profit and opportunity cost while holding the stock. If you have to sell to them, the better scenario would be during trade-in; at least you could try to secure a lower price on your intended piece. A personal sale would be the best bet to hedge against a big haircut.
Thing is dealers can't sell the watch too close to the new price, whether it was one month or not. They are a business, and ask any end user which they would buy, a one month old watch close to the price of a new one or just a new one, it's a no brainer which one you would buy. Even an unworn one cannot be too close to the retail price, so what more a worn watch. I dabble a bit in selling online so the prices goes like that. Any higher you would be earning very little after expenses, or worst yet cannot sell!