Before raid
Put 100k koen in your secure case, this will help you a lot when visiting the merchant.
In raid
You will find the merchant marked on your map. You decide how to play this out but a general recommendation is to loot the outskirts of the map to find good things to trade. You can trade three golds for koen and for the koen you can buy different valued mystery cargos. You can drag around the cargos to hear different sound, these sounds give you an idea of what the mystery item is. You can get purple, gold and even reds from the Merchant.
Cargo sound website
To help with the sounds of the mystery cargo, look at the linked website (HERE!). Several items of different value can make the same sound, with the help of the website you can find the best mystery cargos by ruling out the sounds coming from lower valued items.
Smelter tech
Since the smelter has been buffed, a good tech to use is to loot lower value golds and even purples to smelt. You want the golds that are worth as much as possible, since that will give you more koen to spend at the Merchant.