I do, on occasion, spend money to save money for around the house and yard. I do this by buying Lowes or Home Depot coupons on eBay. These can be purchased for around $1 per 10%-off coupon. And since they can be digital coupons, the same purchased can be used multiple times by printing multiple copies. The key to this is using the coupon at the competitor's store. For example, I recently bought a Lowes coupon for 99 cents. The digital coupon (delivered as PDF attachment via email) had already been used at Lowes so the barcode would not work at Lowes. However, since Home Depot (and vice-versa) accepts Lowes coupons I printed out the coupon and used it to save over $100 on our carpet installation. A few weeks later I used it to save $1.20 of a gallon of deck water sealer. I will be able to use the coupon one more time before the expiration date in a few weeks.
I think the two big box home improvement retails know fully that there is a huge market for accepting their competitor's coupons. In my experience they never bat an eye in accepting these printed digital-coupons. They just accept that they sold more product that their competitor didn't.
I also wondered about the people selling these on eBay, so I sold my coupon back. I listed and sold it for 99 cents. It did not cost anything to list. The buyer paid me via Paypal and Paypal kept 33 cents for their fee. So 66 cents to me. eBay will collect 9 cents for their fee. Total money to me is 55 cents. So sellers have to sell lots of these coupons to make any decent amount of money. And after 50 listing a month eBay charges a listing fee, further biting into any profit. Not a whole lot of sense (or cents) selling these coupons on eBay. But you can save a lot of money using them.