I'm not sure I would recommend that someone do this process to rely on for retirement or anything where adjusting for inflation would be a critical consideration. Although resellers could definitely can (and do) make large incomes ($400K+ gross) and can definitely get more sophisticated with their long-term investment estimates.
Whoever wants to try this method, can chose their own long term investment and make their own assumptions, but the point here is to illustrate what compound interest could do over time.
For anyone who wants to know all of the details, I wrote up a through piece on sourcing, how I picked my items, why I sold low, what my min/max/avg profit was by platform, a list of every single item that sold and for how much, lessons learned, etc here: https://medium.com/the-post-grad-survival-guide/how-i-grew-0-to-1-050-e1a83ad849eb
I probably even included more detail than anyone would want, but it's there for anyone who wants to do this themselves.