Welcome. You really do not need either the EIN or the MSPA certification. Only a couple of MSCs like to see the EIN. Most shoppers just use their SSN. You report MS income and expenses exactly the same way (Schedule C) and under the same rules, for either.
What really counts is signing up with many MSCs and building a reputation as reliable and objective.
Most of all, ENJOY !!
Based in MD, near DC
Shopping from the Carolinas to New York
Have video cam; will travel
Poor customer service? Don't get mad; get video.