I figured that an engineer would prove useful, but then I found out the details of the arrangement... Food is probably one of the best career choices, because of the long time between levels as they increase, and because other jobs (healing, cooking, armor repair) cause hunger in the provider. And as players get more powerful they are less likely to get hit, so armor repair and healing take a back seat to plain ol' hunger.
And high nutrition provides an exp bonus, so everyone who's out for a grind will make sure to fill up before going zombie-stomping, even if they're relatively close to the next level.
Services can also be a bit tricky, because there are so many people trying to get into the moneymaking classes that no one else will ("too many medics!")...
Although services are necessary, they also have the slight problem of competition. Regardless of how much your service is worth, there's always someone charging $5 or lower, and the customer always goes for the lowest price.
Raw goods are treated in a similar fashion, but it's less obvious. There are very few people who will charge under 2x scrap price, and most people will charge closer to 4x. When food is involved, the prices skyrocket.