IMO, the most important trade skill for serious traders is Margin Trading, that's why it's the first skill my traders train to V now. I used to train Retail V and Wholesale IV before Margin Trading V when I was training my first few traders.
Margin Trading is simply amazing for buyers, it allows you to leverage your ISK to place more and bigger buy orders, enabling you to buy a lot more stuff.
It may seem counter-intuitive but I believe it's even more important for new players short on cash than it is for alts of established players who already have plenty of cash. If you have 70 mil ISK it's very well worth spending 35 mil on Margin Trading. It doesn't take very long at all to get Margin Trading high enough to more than double your buying power, so spending half (or even a bit more) of your cash to get the skill is definitely worthwhile.
Of course if you never use buy orders you don't need Margin Trading at all. But if you use buy orders much, it's huge! Train it to V ASAP!