While I like the carpet idea and use one as a temporary 'driveway', please do not use 'used' carpet as it may (and probably does) contain all kinds of cleaning chemicals, and possibly high amounts of baking soda which can mess with your pH, unsanitary flood (septic overflow) residue, flea eggs, or could be from an old meth lab.
Stop by a job site where they are installing new carpet and ask for all the remnants, or make prior arrangement with a carpet installation company ; even new carpeting may contain unwanted chemicals such as a stain-repellents but is better than the unknown items in the used carpets.
Edited for clarification.
