I wish to alter the word this within the url

in category : http://www.example.com/?cat=USA

I would like it to end up like this http://www.example.com/?section=USA

also, from author I wish to change it out to seller
http://www.example.com/?author=1 to

http://www.example.com/?seller=1

How ?

why not only use the custom permalinks like /seller/admin for author section/usa for that category

make use of your .htacess for your mode rewrite

http://corz.org/serv/tricks/htaccess2.php