How do you choose between "trading" a good (locally) and shipping it (abroad) when the two actions have rather different reward structures?
How do the relevant roles you might play (craftsman, captain, trader) affect these choices?
Of course, if you have a warehouse, that gives you a wider choice of whether to trade or ship, and whether to do so this turn or in the future. More to the point, how do you decide, "I need to buy a warehouse THIS turn, for 1-2 turns down the road?"
It depends on several factors that ultimately boil down to What is the roles overall value?
Is it the early game? - In the early game, it is usually more important to gain coins than VP. The reason for this, is that you can exchange coins for buildings that will gain you more coins/VP over the course of the game than you would earn this turn by shipping and taking the VP. Until you have your coin generating buildings setup, usually consisting of a high value trade good/large market/factory, coins are usually of greater value. Some of the expansion building make this distinction moot (Black Market).
Do your opponents make more Coins/VP because of your choice? - Usually, in Puerto Rico, you want to make decisions that benefit you more than your opponents. If choosing Trader will benefit your opponents more, you might want to Captain instead (or select some other role). The few times you might select a role that doesn't seem to immediately help you relative to your opponents, is when you are denying them access to the cash necessary to buy some building (Guild Hall pops up a lot here), or denying them shipping on a large boat (they have lots of corn and you take a large boat and possibly tie up all the boats from emptying).
Does the role have a coin on it - I will many times take a role card that has a coin, because all I need is that extra coin to buy a building I want during the next building phase.
To answer your specific question "Do I need to buy a warehouse THIS turn, for 1-2 turns down the road?" here are my informal steps for evaluating. How many and how many types of goods are you producing? How many and how many types of goods are being produced by others? How many Warehouses are left? What is the opportunity cost to buying the Warehouse?
If you are producing few goods and think you will be able to ship and hold onto what doesn't ship in the next round you can hold off. You can begin planning for a Warehouse purchase and minimizing the hit to your doubloons (i.e., pick up a Quarry, select Builder, Trade what you fear might not ship).
If everyone (including yourself) is producing only enough different types of goods to fill the available ships you need to look at quantities and decide if you will be able to fit your goods onto one of the ships. If you are producing a different good than everyone else and can get it on a ship so that you have guaranteed space for a round or two you don't need a Warehouse immediately.
Small Warehouses are a scarce resource, in the sense that there are not enough for everyone to get one. Large Warehouses cost significant Doubloons and in games I have played I don't ever recall them all being built. Do you think other players will build the Small Warehouses so that you will not be able to? If so build now. Also, there is an opportunity cost in terms of denying or not denying another player the opportunity to build a Warehouse and store their goods.
I have noticed (and we've talked about it in my PR group with mild consensus) that it is easier to make a decision to buy a Warehouse earlier in the game than later. It seems tied to the opportunity cost. Early in the game choices are limited by what can be afforded so it seems less a sacrifice to buy the Small Warehouse. Later in the game it seems as if that higher priced building is more desirable because I've been working toward it so putting off building it to build a Small Warehouse is tough. I tend to try to buy a Small Warehouse early in the game. Sometimes I never need it, sometimes it saves me a lot of otherwise lost goods. The upside of the situation where it is built but never needed is that you get the points at the end of the game. If you build City Hall the Warehouses are violet buildings and will add points that way too.
In the beginning, you need money to invest, so the emphasis is on trading, which generates money. Here, you would want to use the trader and craftsman, all other things being equal.
Later in the game, as it gets closer to the endgame, you are after victory points, so unless you are a builder, the emphasis is on shipping, which generates victory points. Here, the captain is your friend.
There are some other considerations. All goods earn an equal number of victory points when shipped, but some goods are more lucrative than others when traded. In descending order, they are coffee, tobacco, sugar, indigo, and corn. "Comparative advantage" dictates that you want to ship corn more, and trade more coffee.
