There is another thread about arguing the merits of trading verse non trading (http://www.boardgamegeek.com/thread/733451/why-trade-at-all-...)
That is not the purpose of this thread.
The purpose of this thread is to determine the motives for initiating trade.
First let's look at teams and items that they could trade:
For the host:
Infection card - Gain Allies: the object of the host is to infect others, so they would want to initiate trades that get these cards to other people. The draw backs are you only have 3 tries, and if you fail you will be outed, so you want to make your attempts count. Only trade an infection card if you know you are not going to get a gas can back.
Gas Can - Gain Trust: the usual trade appears to be gas can for gas can. Trading one away for another does not net you any difference in your hand, and potentially gets someone to trust you so you can do a later trade. It also lets you know they have a gas can. Unfortunately if they trade you a non-gas can, they now have one and are protected from you.
Any Non-Gas Can - Collect Gas Cans: the host wins if most of the gas cans are out of the hands of humans. Trading an ammo or armor (or energy drink) for a gas can helps get you closer to that goal, and removes a gas can from a human. You have better odds of then infecting them at a later turn.
Gas Can - To find the host: If you have a gas can, force a trade with another human (gas can for gas can). If they give you a gas can, then you know that you both have one. If they try to infect you, you out them. If they give you something else, hopefully you can find another gas can.
Any Non-Gas Can - Collect Gas Cans: If you say you are going to trade gas can for gas can, and then pass a non-gas can item (even if you have some), this now increases your number of gas cans. You need three to destroy the hive. As you were the one initiating the trade, the host might be less inclined to try to infect you, and so this bluff might work.
Any Non-Gas Can - Try to get infected: Even though the game designer says this is not in the spirit of the game, because there is no mechanism disallowing it, it needs to be considered as a strategy. Early in the game, a player may want to be infected, and so they could pass non-gas can cards in order to make that happen.
For any of the above trades I would only state I was trading gas can for gas can, and unless there was a failed infection attempt, I would only say a gas can was traded for a gas can. Letting a potential host know that one player now has fewer gas cans is a bad move.
The second thing to consider is board position.
If you were in a hurry to get somewhere, then if another player is in your path, you would have to trade with them. A host or infected might place their people in locations that force a trade (and they should). This can be avoided with an attack.
The issue is there isn't much of a reason to be in a hurry (unless you have three gas cans and the hive just popped up). The speed of the game is determined by the speed of the players. There is no game mechanism that forces players to go faster. Parasites spawn when players trigger them. The only thing speeding up play is the host and infected. I really can't think of a valid board placement argument for justifying trade. If a player forces a trade because they say they need to get somewhere, I would consider it very suspect.
The third thing is information:
When you know who the host is, and who they traded with, you should know who is safe. You can trade with those people, once the hive appears, in order to get the proper gas cans allocated. The tricky part is the host/infected will do what they can to steal the gas cans. You can also trade ammo to a player without any so that they can attack host/infected.
-Munch "just because someone initiates a trade does not make them a host" Wolf
Good summary - thanks. I'm expecting the game in a couple of days. I just hope my group doesn't over-analyze to the extent of being afraid to trade at all. There's a couple of analytical-types in the group, and after reading this, it seems like there's that possibility. If it doesn't work for them, I'll just have to save it for when I play with my other group, that likes to dramatize things. I'm sure they'll do great with it.