As the druggist, I would have demanded to know what Emily was going to use the poison for. And if she claimed it would be used to kill rats, but I was still suspicious, I might try to contact Tobe or Homer himself to watch her closely to see what she does with the poison.
Emily is a complex character. Much of her psychological problems stem from her troubled relationship with her father. Emily's father had never approved of a possible husband for her. When she starts courting Homer, some of the ladies in town disapprove:
Then some of the ladies began to say that it was a disgrace to the town and a bad example to the young people. The men did not want to interfere, but at last the ladies forced the Baptist minister--Miss Emily's people were Episcopalian--to call upon her.
It seems that her father and these ladies in town are simply against any chance of Emily finding happiness with a husband. The minister also tries to talk Emily out of a marriage with someone of another denomination (even though these are both Christian churches). These women could have been more sympathetic and been happy that Emily had finally found a companion. As a man of God, the minister could have supported the marriage and given his blessing. It seems that all of Emily's attempts to marry were met with disapproval. Surely someone, perhaps her family or cousins, could have offered support and maybe even directed her to a therapist to help her deal with the psychological baggage she still carried from her relationship with her father.