problem using WP_Http with paypal nvp api

Looking at your original code, I’d say you need to add the sslverify arg.

And on a side note, use the HTTP functions available, rather than instantiating the class yourself (you shouldn’t have to load the class manually).

wp_remote_post( $url, array(
    'sslverify' => false, // this is true by default!
    'body' => array( 
        'METHOD'    => $methodName, // if you're using an array, no need to URL encode
        'VERSION'   => $version,
        'PWD'       => $API_Password,
        'USER'      => $API_UserName,
        'SIGNATURE' => $API_Signature
    )
) );