I just played a game with this set:
Baron, Counting House, Fool's Gold, Grand Market, Haven, Jester, Loan, Noble Brigand, Spy, and Trading Post.
I opened 5/2 and my opponent 4/3, which is really really unfair here. The game was sort of immediately over, so that's not what this post is about.
I can play Trading Post into Grand Market, and WW just can not. Dominion isn't always fair, especially if you look at a single game. But oh well, I was on the short end of a 5/2 opening a game later and losing to that.
However,
in this game, the play I'm proud of is on turn 6, where I get to $6 for the first time (unfortunately with coppers).
In my opinion, the best buy is now a Spy. Second best buy would be nothing.
I don't want to buy a Jester, because if I flip a fool's gold (not unlikely) I have no clue who to give it to. But I guess it would be an option after a few spies, because inserting a few curses/extra coppers into his deck is also quite good.
I don't want to buy a gold, because all that I'm interested in is increasing the frequency of my Trading Post. And once I start drawing Grand Markets, I want to draw them all, not some silly gold that's in the way.
I don't want a second Trading Post, because they would only clash and annoy each other by taking the spot of a copper/estate that wants to be trashed.