How did the Romans actually do any mathematical calculations with Roman numerals? Without the concept of places (units, tens, etc.) how did they add, subtract, multiply, divide, sell slaves, and ...

Roman numerals were fine for adding and subtracting. Fibonacci saw that complex math required a better system.

In his book The Joy of x, Cornell University math professor Steven H. Strogatz writes, "Unfortunately Roman numerals creaked and groaned when faced with anything larger than a few thousand.