- If I have a resource, can I trade that resource with as many interested nations as I want?
Trade goods grant a passive income boost to all trade agreements; if you wish to use it or trade it away, it can grant specific bonuses (copper/bronze will make bronze-age units cheaper, gold and gems will improve public order, stone will decrease the cost of building, etc.)
By default, you will passively trade it; if you wish to use it for a specific purpose (either to trade it away or for a bonus; name what bonus you're aiming at - some trade goods have multiple bonuses they can get you; EX: Coffee can be supplied to the lower classes to increase productivity, or supplied to the upper classes to increase research and rate of advisers), make sure to state it and I will add it to your modifiers.
- How unilateral is trade? Can I declare as an action that I start a trade route with a certain nation, or should that be handled in a trade treaty?
Both nations must agree to a trade agreement, unless it is forced (if you start a trade route without the other nation's agreement, either secretly [selling to the underworld] or as a peace deal, you will receive bonus econ, and they will receive either none or negative econ).
- Is there a hard limit on # of trade routes?
Two for non-trade nations. After this, it will begin to cost diplomatic power per turn. You have an additional max trade route before the penalty as a trade nation. National ideas, advisers, and infrastructure can all increase this max.
- Do players have to have a land border to allow trade? Does having a nation allow sea trade to occur, or does more research need to take place?
If each player has a coastal city, no land border is required. Also, if you both have a trade agreement with a nation between you, you can form a trade agreement with each other, port or no. Sea trade tends to be horribly inefficient (think 1/3 efficiency) until Sailing and more advanced research is gotten.