Tax collectors don't get to keep the taxes they chase up. They are paid by a third party to perform their task. Hunter-gatherers were not paid by anyone to hunter-gather, they had to perform that task in order for survival. Prostitution is not needed for survival (in the animal kingdom it is considered prostitution when it happens outside of procreation and there is a service exchanged for it, shelter/protection/food).
Everyone was a hunter-gatherer pre-history, hence prostitution being the world's oldest profession. The first time someone exchanged sexual favours for goods was the first time someone provided a service for which they were "paid".
Think of it this way: You go out, acquire food, cook the food and eat the food but that isn't your profession. It's survival. Your profession is some other task for which you are provided the means to acquire food.