Is it rare?
Is the piece one of 100, or one of 10,000? The difference is key.Just be careful not to confuse rare with undesirable. If you only find one on the market and the price is low, it’s more likely that its the latter.
What condition is it in?
The pre-owned / second hand watch market is a lot like the used /second hand car market. Ask pointed questions about what maintenance the watch has undergone so you don’t get stuck with those problems.
What are your watches complications?
Watches can have a lot of bells and whistles, including perpetual calendars, minute repeaters, and power reserve functions. What, besides telling time, does the watch do? The more complications, the more it’s worth. That means that just because you find a certain functionality to be excessive, doesn’t mean it should be overlooked.