Since ZestMoney gives you money upfront and hopes that you repay it back in EMIs, the more obvious trust issue is the one in the reverse direction i.e. Can ZestMoney trust you?
I had a quick glance at the workflow of a ZestMoney payment. You place your order directly with a Seller XYZ who accepts ZestMoney as a mode of payment. Since you don’t place your order with ZestMoney, the question of you trusting ZestMoney doesn’t even arise.
If the real question is whether you can trust the Sellers that accept ZestMoney, that’s a valid question. I couldn’t recognize a single seller on the ZestMoney merchant list. To me, that’s a red flag. But others may be more adventurous than me in their online shopping endeavors.
Personally, ZestMoney or no ZestMoney, online or offline, Caveat Emptor is a very useful principle to keep in mind whenever I buy anything from anyone.