Buying and selling goods is actually pretty straightforward once you know how the formulas work!

Buying

To calculate the cost __per unit__ when buying goods the
following formula is used (see 6.6.1):

So the cost to buy one unit of Gold (Goods Factor 29 - see 6.6.3) from a trade city with a rate of 6, and with 100 units currently in the city would be:

or...

or 26,540 Louisdore per unit. So to buy 50 units would cost 50 x 26,540 = 1,327,000 Louisdore!

Selling

Selling goods is much the same, with just one added complication. The formula is (see 6.6.2):

So the price for selling one unit of food (Goods Factor of 4) at a trade city with a rate of 5, with 1000 units currently in the city and selling a total of 500 units would be:

or...

or 554 Louisdore per unit. The grand total when selling 500 units of food would be 554x500 = 277,000 Louisdore.

I hope this clears this up for you!