How do get boxes to ship books at amazon or things at ebay?

Are you talking about general cardboard boxes? If you are try your local grocery storys those are free otherwise you will have to pay money for the boxes at the post office.

Just trying to think of an inexpensive way for you to go about this.

The only boxes provided free of charge by the USPS are Priority Mail boxes and supplies for Priority Mail (tape, labels, etc.) They do not provide boxes for Media Mail, which is what you would want to use to ship books (slower shipping, but the cheapest way to go).

I would not recommend purchasing any boxes, as that eats into your profits. Check local stores for boxes, especially dollar-type stores. They generally have a variety of boxes of all sizes.

I get a nice variety of small boxes at a local convenience store. After awhile, they just started saving them for me.

I use bubble mailers for most of my orders. They aren't free like boxes but most sellers seem to prefer them. I just look around various websites and can get mailers for about 20-30 cents a piece

It all depends on what your profit margin is, or what you want it to be. I've purchased bubble mailers for books in the past, when I was selling a large number of books - but my profit margin dropped dramatically, as books don't generally sell for a high dollar amount on Ebay - depending on what they are. If you have something rare or antique and in excellent shape, you can do well with it - but if it's rare or antique, you'll want to protect it better during shipping than using a bubble mailer.