Do I need to ask for a higher salary since I worked as a cashier at the same time I'm also the purchaser of our company?

yes, you have to do multi task

If you can prove your self and your skills, you may ask for higher salary. At early stages i dont think so a company will offer you higher than the market.

Sure you can...cashiers are eligible for risk allowance..since you are dealing with cash...

Not a good reason to ask for higher salary. When you work with an organization, you have to be a multi tasker as no one is really doing a single job. To ask for a higher salary you should rather increase your credibility within the organization. Organization should recognize that you are doing something extra and doing it nicely,you will get the reward by itself. If you feel more pressure when doing two jobs simultaneously, you can always ask for a helping hand.  

sure ..but at the right time