NO BLATANT PIRACY.
It's not nice to make and sell cartridges of recent homebrew games unless the author allows it. But fortunately, some authors do allow it.
If you just want to learn how to make a repro:
Start with Holy Mapperel (formerly Holy Diver Batman), a test ROM designed to work on several common mappers.
If you want ideas for things to make:
The Homebrew Projects forum has a topic listing games that can be reproduced and sold.
It's not nice to make and sell cartridges of recent homebrew games unless the author allows it. But fortunately, some authors do allow it.
If you just want to learn how to make a repro:
Start with Holy Mapperel (formerly Holy Diver Batman), a test ROM designed to work on several common mappers.
If you want ideas for things to make:
The Homebrew Projects forum has a topic listing games that can be reproduced and sold.