you can give a gift to merchant so you can learn and level up mercantile skill from them. if you buy Her Vanishing Silhouette dlc you can combine your skills into one , you got the skill book from your combined skill then sell it into auction, oh yeah only sell it to Night auction because it can make a lot of money than sell it to glittering house.

i got 600-900 guan from selling top quality skill book.

there's a lot of methods to gain money fast without using mods. marrying duke sister for example you got 2500 wen/day (correct me if im wrong) but the downside is it reduce gift buff to other female.