How to override a plugins script

In principal, unless there are documented APIs and/or filters to the plugin, you should not do anything that is more then using core hooks if you want to be able to upgrade it in the future.

If no appropriate thing is available, you should contact the author and ask for it, or decide that you are taking full responsibility and just fork it.

In your specific case, you might be able to change the attribute after the initialization of the object, but next version this attribute might not be there any more.

Edit:
since there is a filter you just need to use it like

function wpse_242093_auctions($auctions) {
  return array('new' => __('Brand New', 'wc_simple_auctions'), 'used' => __('1/10', 'wc_simple_auctions'),
            '2/10'=> __('2/10', 'wc_simple_auctions'), '3/10'=> __('3/10', 'wc_simple_auctions'), '4/10'=> __('4/10', 'wc_simple_auctions'), '5/10'=> __('2/10', 'wc_simple_auctions'), '6/10'=> __('6/10', 'wc_simple_auctions'), '7/10'=> __('7/10', 'wc_simple_auctions'), '8/10'=> __('8/10', 'wc_simple_auctions'), '9/10'=> __('9/10', 'wc_simple_auctions'), '10/10'=> __('10/10', 'wc_simple_auctions'));
}

add_filter('simple_auction_item_condition','wpse_242093_auctions');