You just need to make her go into debt by selling stuff to her and making her contract buyable, then raise her obedience after you get her to the point where you can suggest she be responsible enough to be an on-board merchant.
Cunt/cock tease. Make sure your lust isn't too high during the 40-80% stretch, otherwise she'll take control and actually drop obedience. Or let her lust hit 100 and then punish her.
Just do the Cunt/cock tease while you both have 33+ lust and she is on 80. Then you lock her in, talk to her about everything get a 5th talk option and encourage her to be a merchant but you need to give her a loan of 15k.
Seras wiki has all the details. :3 https://wiki.smutosaur.us/TiTS/Sera#Talk