Listed prices can be all over the place, but you can find out what watches have sold for in the past by:

• Searching completed auctions and sales on eBay
• Searching sold items on /r/watchexchange and other places people sell watches
• Seeing the average price of a watch on Chrono24 and other high profile places to buy and sell watches

The value of a used watch is, however, highly dependent on its condition.

You can generally assume that the full retail price of a watch is the highest price of it, unless it is a particularly valuable luxury watch or another special case, and that the "regular" price of it is likely 10-25% lower in a competitive market, and that the used price is 50% or lower.

These are just guidelines based on what I see, especially for watches which can be bought new. Watches with supply shortages or which are discontinued but in high demand can be worth more than a normal used watch.