Things They Should Invent: people who returned X kept Y

Retailers keep track of returns - you almost always have to give your name and contact information when you return something.

Retailers also frequently keep track of purchases, using loyalty cards and customer accounts and such.

Therefore, records must exist about about what customers tend to buy and keep after returning a given item.

They should use this data to make things easier for customers!

For example, if a lot of people exchange a given garment for a larger size, that's a sign that the garment runs small.  If a lot of people return Brand X and then buy Brand Y, that's a sign Brand Y might meet your needs if Brand X doesn't.

I've previous advocated for a universal clothing sizing wiki, so you can find what else fits people who fit the same clothes as you. Analyzing clothing return data may well serve a good part of this function, without requiring diligence from a large number of individuals.

At an absolute minimum, I'm sure Amazon already has this data somewhere in their massive stores of data they collect on everyone.  They already have a "people who bought X also bought Y" algorithm, so perhaps when they see you returning something, they could also add a "people who returned X kept Y" feature.