You would probably need to have a portfolio of approx. 1,000 .com domains. If you calculate with a low sell through rate of 1%, you would sell about 10 domains in one year.
With hand reg or drop reg domains you could achieve an average sale price of 1,000 USD per domain. That means your income would be 10,000 USD.
Your cost for regging 1,000 .com domains would be about 10,000 USD. That means you would reach break even.
Now you have to calculate your time for finding and managing 1,000 domains and all other costs involved. Also the cost for renewals in the coming years. You will fast see that you're loosing money.
How to come out of this situation? You need better domains. That means you will have to buy better domains. Lets say 1,000 domains with average purchase price of 50 USD. That means to invest 50,000 USD in your portfolio.
This names will probably fetch higher sale prices and you will achieve a better sell through ratio - maybe 2%.
That means 20 names with average sale price of 2,500 USD. You will again reach break even with 50,000 USD income. Again your time and all related costs are not covered - you lost money again. The question is if your able to find 1,000 good domains for 50 USD each.
If you buy 100 USD domains you will probably reach your goal easier. But you would need to invest 100,000 USD.
What I want to say is - you have to invest serious money and time to get repeatable sales over a longer period.
Sorry, there is no easy money here.