I'm no expert and don't know the full extent of what's already happening and what this ban would change, but I would say there is evidence that this is already happening.
In a recent episode of Clive Myrie's African Adventure where he goes to Ghana, he "heads to one of the world’s biggest second-hand markets to meet the designers giving discarded clothes a second chance".
They show a lady that bought a "crate" of random unsold clothes for around 500 USD, and she prays before opening it hoping it will contain clothes in good condition she can resell. The show claims that on a "good day" she can make something like 50 USD on such a crate.
They also (very) briefly show a huge landfill of what appear to be discarded clothes.
Keep in mind that this is only an entertainment show, so this is most likely only the tip of the iceberg.