Wow, these post is for the strong in mind and it requires wisdom @lordjames. But, one after the other I will tackle it. Starting with these,

Is it justifiable to press down prices with the poor when you can afford to pay at a better price because you know they have almost no other choice but to sell at any price just to survive?

These, question has two answers. It's has a NO and a YES answer.

NO
I say it's not right, because most of them only have these as there source of living and nothing else. Just to eat and survive and anytime it's time for them to leave the world they would.

They look to the rich people, and the government as a source of lively hood to them. They believe that, they are a little angels sent to help them by buying there goods from them. And when they are cheated, they feel betrayed and unhappy and God won't be happy with us, instead buy it at the prize they require, even if we prize it shouldn't be to our own gain.

Cause, like it or not the poor would always be in our midst and we the wealth, we are given a responsibility to use our wealth to affect lives in a positive manner.

YES

In business, it's always good and it pays to be honest in business. Honesty in everything you or we do makes us outstanding.

Sometimes, the poor has a way of hyping the price of goods when they know that, it's not what the price seems to be. If a bunch of bananas cost $3 and you know that very well, cause you have been buying it that way.

And you do hear a different price from what you know. You will be forced to beat down the prize to the normal price you know. So that's my own opinion. @lordjames