If one has no surplus at the end of the khums year, rather in the negative but he has a few items liable for khums,(gas in car, clothes he hasn't worn for that year etc) does he have to pay khums on these items?

If no cash amount remains with you as a surplus amount at the end of the year but still you have solid assets as a surplus, you are entitled to pay Khums for that thing after evaluating it but if you are not able to pay its value in cash it will remain as a dues on you and later on you have to pay it’s value.