Yes, it is legal for you to sell the coupon.
There are currently thousands and thousands of coupons for sale on eBay.
The problem you will probably run into is finding a buyer. Coupons
generally sell for less than they are worth. A 15% off coupon's value
would depend on how rare coupons for that store are, and connecting
with a buyer who is planning on making a large purchase.
eBay is the quickest and best place to sell your coupon. After listing
it on eBay, you might want to find some online forums that deal with
the store and advertise the coupon there (make sure that it is ok to
do so, some forums will not allow this).
I hope that this answers the question to your satisfaction. Please let
me know if you require further information and I will do my best to
further assist you.
All the best,
also used personal experience