Much of this was already said by others.
When I read your original question, my first reaction was, "There's not enough information here to answer the question. I need to know if $50K is the typical preowned dealer's ASKING price -- like what I'd find if I looked up this watch on Chrono24 -- or if it's the typical (lower) SELLING price." That's hugely important. Also, my definition of "reasonable" (irrationally) involves what I originally paid for the watch, regardless of when I bought it: I tend to fixate on what I paid, and I hate taking less than that (if I bought it preowned). Lastly and more generally, when dealing with real estate brokers and others who sell things on my behalf, I avoid subjective words like "reasonable" in my instructions to them because I don't want to waste their time or mine. Instead I tell them, "Don't even bother to contact me with offers below $__."
All of that being said...
In this case, I'll assume (a) $50,000 is the typical preowned dealer's asking price, (b) the watch is not especially rare (so there are many examples for buyers to choose from), and (c) I paid $45,000* for the watch. Under those circumstances, I'd probably sell it for $45K in the near future, and then increasingly take lower offers as time progresses, maybe even going as low as $42K.