I use reputable sellers on ebay often. My local stores are cracking down on the usage of printable coupons (my Kroger now only allows 2 per order). I only buy coupons for things I use quickly and/or things that have a longer shelf life (like toilet paper, hair dye, etc.).
I get the paper on Sunday and look through for the sales that week. If I see a really good deal on the hair dye I use, for example, I get on ebay the same day and buy coupons for $.50-$1 (usually this gets about 10 or 20 item specific coupons). Most sellers are very fast at shipping so I get them in 2 days and can use them before the item goes off sale and I go and stock up. I buy hair dye that's normally $8-10 a box for $2-3 a box.
I also do this for restaurant coupons if I know I'm going somewhere later that week to eat or for Home Depot/Sears/Lowe's if I'm buying a big item. Just be sure to read the fine print and when the coupon actually expires.