When you buy STEEM you need to sell SBD and when you buy SBD you need to sell STEEM for it. Maybe I should change the labels and field order to make it a bit clearer. I will think about it when I find time in the coming weeks.
Yes, that's right. Maybe it's the labeling of "sell amount" field in the "buy" tab and the related field in the "sell" tab what confuses me. Don't know if this happens to other people.
I'm usted to think of pairs so when you buy you can set the amount to use of your current balance of the correlated pair. You cannot trade a currency pair if you haven't any currencies of a pair.
Edited: "You also need the market orders(selling, buying) book so you can select the price to buy or sell".
Hope I've managed to explain It better so It can be of some help.