Having a merchant guild in the province the merchant was recruited may gain your merchant traits. Acquiring other merchant's assets will gain your merchant traits. Going through other faction's lands/trading in other faction's lands will gain your merchant traits. Monopolizing upon any resource that has at least two of that resource gains your merchant traits. And of course failing in any losses causes negative traits.

By the way, just a pointer for the future. This is probably best asked in the M2TW singleplayer citadel area of the forums.

Tschüß!
Erich