I presume you have two check boxes side by side and you want new or used checked not both. If thats the case have you any objections to using radio buttons?
Yes I have several of these that only one or the other needs to be checked.... I have no objections to anything.....I am sorta new at these. If someone would walk me through I have no objections to any ideas as long as it works! LOL
Thinking about it, you don't need to amalgamate the two fields, they both contain the same data. If its "New" it's not "Used" and visa versa. So you only need one of the fields to store the data.