Was American slavery economically advantageous primarily due to being able to also sell offspring?
No, American slavery was economically advantageous regardless of selling offsprings.
If X continuously makes revenue breeding slaves and selling them to Y, we can assume following:
- Y makes revenue (directly or indirectly) by using slaves bought from X (otherwise Y would stop buy slaves);
- In average, this revenue made by Y exceeds amount which Y pays to X for the slaves, otherwise Y would stop buy slaves;
- X can not effectively make revenue by using slaves the same way as Y, otherwise X would stop selling slaves to Y and start to use them himself, making more revenue (see 2).
All these conditions were in effect in US by the middle of XIX century.
Let's read and analyze "A Declaration of the Immediate Causes which Induce and Justify the Secession of the State of Mississippi from the Federal Union", second paragraph:
"Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world. Its labor supplies the product which constitutes by far the largest and most important portions of commerce of the earth. These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun. These products have become necessities of the world, and a blow at slavery is a blow at commerce and civilization"
So we have Lower South states (Y), making revenue by producing "product which constitutes by far the largest and most important portions of commerce of the earth" (cotton), using slave labor.
We have Upper South states (X), who can not produce cotton, "peculiar to the climate verging on the tropical regions", as effectively as Lower South (condition 3), but can sell slaves to the Lower South states.
As we can see, American slavery was economically advantageous primarily due to unique ability of the Lower South states (Cotton kingdom) to supply "the product which constitutes by far the largest and most important portions of commerce of the earth". Selling slave offsprings (from Upper to Lower South) can somewhat increase this advantage, but could not be its primary reason.
Speaking about Chinese laborers - this is pointless discussion. By 1860, it was about 35000 Chinese people in US, 100 times less than number of slaves. By 1850 - about 4000, practically none - before 1849 Gold Rush.