Other contributors have said "Traders were part of what society class?" is the same question as "What was the lower class in Roman society?" If you believe that these are not asking the same thing and should be answered differently, click here
Proles.
The proletarians were those who had no property - all they contributed to the state was children, and the word means breeders.
Note on Plebs:
Plebeians were simply those who were not...
Initially the Plebeians were the lower class of Romans. However, as times changed and the city grew, the Plebeians power, wealth and class also grew. During the mid to late republic the proletariat,...
The wealthiest class of Roman society was more than likely the equites, if you take the class as a whole. They were the businessmen of the empire and usually made huge profits. There were also...