Read the wiki in the link.
Log into Bing.com.
Click on shopping.
Type roomba in the search box.(Only certain search terms will give the 20% link.)
Click on the ebay link on the right side of the page.
That will take you to ebay. You will see the 20% cashback logo at the top of the screen.
Use Buy-It-Now on any auction and you will see the 20% cashback and cashback amount on the review page before purchase.
Other 20% search terms
roomba (20% as of 11:30 pm PST)
samsung
apple
netbook (still working @ 20%, 10:19pm PST)
netbook (still working @ 20%, 10:00am EST)
laptop
hp
vizio
western digital
plasma
dvd
jewelry
panasonic lcd
golf clubs
cheap wii (worked at 0935 CST 12/7)
fuji camera(worked at 8:37 am 12/7)