Hi there! Great question @Godmother23, and welcome to the community!
There are actually a few reasons why we don't allow coupons or gift cards. The main reason being that we can't verify the authenticity of the coupon. Additionally, most coupons say "Not for relale, no cash value."
Generally, selling coupons on any platform is in violation of the "non-transferability clause" and redeeming a purchased coupon is basically redemption fraud. In addition, there are way too many conterfeit coupons and/or are only applicable in specific cities/areas.
Yes, there are sites that serve the sole purpose of selling coupons, but they have bypassed the "non-transferability clause" by claiming to sell services/subscription, rather than the coupon itself.
eBay allows some coupons to be sold, but the premise is that the actual "paper" is what is being sold (under eBay rules), and not the redemption value. It also has specific restrictions on how it can be sold.
If you want to offer the coupons for "free", post them on CL.