Understanding my predecessor

I can see why my predecessor quit after a year. He did more work than me by himself. I was told he often worked 6 days a week. Today I spent 4 hours making a spreasheet for a group cause their invoice was over 200 pages. They had 2 paragraphs of problems the sales team refuse to answer and instead pushed it on me. "Let us know when you fix everything" was the phrase in the email. Excuse me, why can't they answer questions and fix problems. I don't have access to sales contracts to know what the group was told or what rates should be. I'm getting tired of being the sales department's secretary. I even lashed out at my roommate who didn't deserve it and apologized with a hug. I don't want to quit. I love my job itself, some of the coworkers and managers are driving me crazy.