If you don't know the person, or aren't sure you will get what you paid for (for example, items bought from an online bidding or sales site), you should not use Zelle for these types of transactions.
These transactions are potentially high risk (just like sending cash to a person you don't know is high risk). Neither Ascend nor Zelle offers a protection program for any authorized payments made with Zelle – for example, if you do not receive the item you paid for or the item is not as described or as you expected.