How do used book sellers afford to sell books so cheap? How are they affording postage?

Typically, when I purchase a used book on ebay, I look for books that are less than $5.

I have a large collection of books that would be expensive brand new, but I buy them used in good condition for about $4.00 on average per book.

My question is how do seller afford this, because I'm looking at the cost of USPS postage for Media Mail, and it starts at $4.13 just for postage. Then if you add in the cost of a polymailer bag, it's like 25 cents, so that comes to about $4.38 just to mail the book. That just doesn't make sense.

The reason I am asking is because all these used books that I have read, I wanted to post them back on ebay and resell them. But it doesn't make any financial sense if I am paying $4.13 for postage.

Can someone help me figure out how people are selling these used books so cheap?

I'm on ebay right now, and I see one book is used, good condition, and it's $3.68 with free shipping.

I don't understand how sellers are doing this.