I have the exact same thing for sale, a booklet of 4x$100 coupons ($400 total value) for United Airlines, that I got after being bumped off a flight last year. They expire on May 16 (one week from now), so if you are interested, act fast! The reservation needs to be made by that date, but travel can take place anytime afterwards. I am asking $300. You need the physical vouchers to get the savings; I can mail them to you, or you can pick them up if you live near Woodmere, NY.
I also have a $75 United Airlines e-voucher that expires on May 12, but for which travel must be by that date, so it will only be useful if you are about to take a flight.