What is the term for when someone buys something to wear once, and then returns it?

Solution 1:

It's called wardrobing

Wardrobing is a form of return fraud where an item is purchased, used, and then returned it to the store for a refund.1 It is most often done with expensive clothing - hence the name - but the practice is also common with tools, electronics, and even computers.

It's discussed in this Quora thread

Is it common for people to buy clothes, wear them once, and return them?

Solution 2:

From the Wikipedia page on return fraud:

Perhaps the most well-known form of this abuse is "wardrobing" or "free renting" – in which the person makes a purchase, uses the product(s), and then returns the merchandise.

Another source also mentions the term “deshopping”.