To sell Safaricom airtime or credit through scratch cards then you have to be an authorized Safaricom dealer. You will get paid through the profits you generate from your bulk airtime sales.
You can get a profit of about Kenya Shillings 5 for Ksh. 100 airtime. Do the calculations for the rest.
Selling Safaricom airtime through scratch cards is a business that is in decline since many people these days prefer buying airtime through Mpesa. You should do your research well for market demand before venturing into the business.
Alternatively, you can check: How to sell Safaricom airtime through Pochi La biashara (you do not have to be a Safaricom authorized dealer)