It takes awhile but is quite gratifying; sometimes even more gratifying than the acquisition.
First step is to id which ones to let go.. and that takes long for me. Mind vs heart thing and all that. It takes years for me. Then the opportunity usually follows.
I also take note of low hanging fruit situations. If I see an opportunity to sell requiring almost no effort, I take it as a sign. But you need to do the first step so that your mind is conditioned to spot opportunities.
In your case, I’d probably just buy the Cellini first and worry about selling other pieces later not unless your dealer gives you an opportunity for a good trade.