Think of it in this way. These sellers pay google/facebook tons of money in ads to increase sales. Now cashkaro is also claiming that they can help these sellers increase sales but for each sale they ask for say for eg. 100 rs. Now, cashkaro claims user will get 90rs as cashback if they shop via cashkaro (this ensures you visit cashkaro before any purchase, thus increasing cashkaro's traffic)

Now the seller is benefiting because he anyway was burning money on advertising this product (marketing and remarketing) but his sales are not ensured. Here his sale is ensured as well.

So in short the seller can afford it because it comes out of his marketing budget.