Check eBay, Chrono24, and the sales sections of watch forums for recent prices. Regarding servicing a watch just before selling; I would advise against it as you're unlikely to ever recoup the total cost, and everyone everywhere has their preferred watchmaker. Personally, I'd wear it until you're satisfied you've got your money's worth out of servicing it and then sell it on. Many people are happy to buy a watch serviced in the past few years and wear it.