I live in a poor country. People here buy "American clothes" which importers get inside "pacas" (random bundles). Those clothes come USED from rich countries.

My assumption is these clothes are dumped to someone to get rid of them, and then that person bundles them and ships them to poor countries. Once here, someone buys the bundles, sort the content according to their expected retail price and sells them to resellers.

There is junk that can't be sold and is destroyed. Except in some cases, like in Chile, where they are just dumping the used junk "intact" in the desert.

Prohibiting destroying new clothes is a net positive. There is market for clothes in poor countries, but it is already being exploited. Some clothes will always be dumped in poor countries, but not all of it can be resold. The manufacturers will make less clothes, there is no way around it.