It is true that the importance cannot be determined here because the qualifications and skills required for a sales executive and a sales cashier are different. For example, if you are running a textile shop, the job specifications would vary depending on the sales turnover and customer segment.
Differences in Qualifications and Skills
In general, for a sales executive, one may prefer more customer interactive experience on the marketing side, whereas for a sales cashier, a preference may be for more finance-related bookkeeping knowledge or experience with payment accounting software.
It all depends on various factors that cannot be summed up in a short note for determining importance.