The first time in Baldur's Gate II Chapter 8 when you enter the Smuggler's Cave in Amkethran, you will see some of Balthazar's monks shutting the smugglers' business down. Regardless of whether you help the smugglers or not, you can trade with Carras, although you can only do it once. If you don't enter the cave in Chapter 8, you will have missed your chance, even if you go in the Smuggler's Cave in Chapter 9.
Helping the smugglers will make Carras give you a discount. He then will sell for only 99% of the regular item value (opposed to 140% without discount) and buy items for a rate of 80% of their value (30% without discount). With high reputation and Charisma this can lead to a situation where a unique, high valued item, e.g. Enkidu's Full Plate from his own stock, can be sold back to him for more coin than it did cost when buying – effectively letting Gorion's Ward gain coin through trade, here.