Important Skills for Trade

Trade
without this skill you are useless as a trader. You need to train this up to level 4 I think to get access to the more advanced trade skills. Each level grants you 4 more outstanding orders. Level 4 I think is the highest level I recommend until you decide otherwise in your advanced career.

Marketting
If you have an item in a station other than the one you are currently in, you need Marketting to sell it. Otherwise you have to get into your spaceship, fly to that station so that you may sell that item. Marketting allows you to do that without being forced to fly there.

Procurement
does the same thing as marketting except it is for buy orders rather than sales. It lets you place a buy order in a distant station. Note: Anyone can by something for sale anywhere in their region without Procurement. Procurement only affects placing a "BUY ORDER" it does not affect buying something that is already for sale.

Day Trading
Like Procurement and Marketing, this skill lets you "adjust" the price of orders without having to be docked at the same station as the order.

Margin Trading
lets you place a buy order even if you don't have the full amount handy. This is useful if say you want to buy 10,000 units of tritanium, but you only have the money for 7,500 units. You can still place the buy order, and while it is slowly being filled you have time to make some more money to cover the purchase. Note: you still need to have the money at the time the buy is executed, otherwise the transaction simply fails, the buy order is cancelled, and you loose your broker's fees for the full transaction. As a larger buyer, I often place many buy orders across several systems and regions knowing that I will not be able to cover all the purchases. I however have guaranteed that I will get my orders filled to whatever I "can" afford regardless of whether business is slower in one area rather than another. The broker's fees are a small fraction of my profits, and I can afford to pay those extra broker's fees as a crude form of insurance that my buys get filled somewhere.

Retail
Once you have trade to lvl4, you need to train in Retail, this increases the number of outstanding orders you can manage at once. Increases your number of outstanding orders by 8 for each level.

Visibility
When you are placing an order at the same station as the order, you can by default choose how far away you want people to be able to fill your order. For instance, if you place an order for 10,000 tritanium and set the range to Region, you will soon discover that you have a few units of tritanium scattered all over the region and you have to go pick them up. Seems like a pain in the neck and it is, but setting range is also quite useful when you only have space for one more order, and you want people in several nearby stations to be able to fill this order. Visibility provides the ability to set this range in exactly the same way but while you are setting up an order from a remote location, just procurement allows you to create remote buy orders, visibility allows you to set the range of remote orders.

Accounting
Not a "necessary" skill except that it allows you to train for Margin Trading. Accounting has no real short term value, but over time it will save your lots of ISK that would otherwise be eaten as taxes.

Broker Relations
Also not very important, it does not enable an advanced skills, but also over time can save you lots of money. Until you get into really big trades, this skill is too expensive to be worth it. ROI on this skills can be measured in month and even years unless you trade hundreds of millions of ISK per trade.