i'm going to post a book around 1KG. how should i pack it so that poslaju would accept it?

use a box a bit bigger than the book and wrap in those brown packing paper i guess.

can i just wrap it using brown packing paper?

you can but you have the risk of damaging the book on delivery coz those mail services tends to stack and throw stuff i guess. but that's your choice man.

ya .. better put in the box .. if not later when reach the buyer hand.. it will become tissue paper.. .hahahaa..

put in a box...
wrap with white or brown color paper oso can...

If cant find a suitable box use bubble wrap then wrap with brown/white paper

yup. box ensure the quality of ur books

Where can I buy these boxes? I don't mean those fancy boxes you buy at gift stores (too damn expensive). Just the standard industry grade boxes.

You can purchase it at pos office.

yup. there's box at postoffice called pospak. cheap one can ask kakak at counter ~