How does the used gear program work? Do they take back used stuff from other places, or just returns from purchases made directly from REI?

Yup, the resell the used REI products that have been returned at a discounted price.

With a very liberal return policy REI often has a large inventory of used or lightly used items.

It's all from returns of REI purchases.

I emailed the used gear through the website, but I'm sure they get a ton of emails everyday and have not gotten back to me.

Unsure