Above is the trade screen for Merchant of the Skies

1. You money
How much money your character has and is the primary source of funds for all financial transactions.

2. Your ship
What goods is on board your ship.

3. Buying
This is where goods you will be buying will be placed prior to confirming the trade.
Note: You can only trade a maximum of six stacks of goods.

4. Selling
This is where goods you will be selling will be placed prior to confirming the trade.
Note: You can only trade a maximum of six stacks of goods.

5. Merchant
What goods the merchant has available. The numbers shown indicate exactly how much of each good is available.

6. Trade Post
A generalised indication of how much the goods are going for. Coins range from 1 to 5.

7. Trade
How much tax will be charged on trades, which is taken off the sale price. Only the final, post tax, price is shown in the selling box (if no purchases are being made.)

Or: What do the coins mean?
All basic goods offered by a merchant have a number of coins next to them, this indicates their relative price.

1 Coin: Cheap
This is the lowest amount that can be charged for this good, a great purchase price for you.

2 Coins: Slightly Under Price
Cheaper than normal and a decent purchase price. Be aware you can still lose money on trades due to tax.

3 Coins: Average
The normal price charged for this good. Only profitable if being purchased and sold for five coins, or selling one coin purchased goods.

4 Coins: Expensive
Goods are being sold for more than normal price. This should be your normal minimum selling point for decent profits.

5 Coins: Very Expensive
Goods are massively over-priced than normal. Perfect for selling anything from 1-3 coins, although selling anything picked up for 1 coin is perfect.

This is the classic buy low and sell high.

The ideal is to purchase a good listed for one coin and selling at five coins, but at the very minimum you are looking for a two coin difference in prices to make any form of profit.
Or in other words your minimum targets are;
But in all honesty if you aren't at least managing 1:4 or 2:5 coin trades you will find it very difficult to make money.