As has been said, what do you have to sell and what are you looking to buy.
Nothing else to be said.

I must admit I prefer p-ex to a private sale becasue, if I am honest, I don't want all the faff that goes with a private sale. However this does cost me money, no question about that. For me the experience and the ease of a trade with a dealer makes it sensible, also the relationship building which puts me in the way of watches I want occasionally.