Selling Safaricom airtime (scratch cards) is a business that is in decline because many Safaricom users these days prefer buying airtime through their Mpesa accounts. You should do market research well before venturing into the business.
You also have to be an authorized Safaricom dealer to sell Safaricom airtime through scratch cards.
You might want to consider selling Safaricom airtime via Mpesa using via Pochi La Biashara and get paid a 5% commission.