After having read how the coupons that do not work on sale items work in this thread:
http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2802992 I won't be using any of them in future. 

I strongly believe that the merchants here who sell high quality products should be able to set their own prices for their quality work.  The marketplace is already seeing a serious decline in the average quality of products and this new type of coupon will make that worse.