I have seen this in the past and I do not know exactly what triggers it. Shortly after paying for an ebay item through paypal today a 10% off coupon appeared in my ebay profile. It is good for 48 hours and up to $100 off (i.e. you buy something shown at $1000 and you get charged $900). The last time one of these appeared for me I did use it towards a large purchase and got almost the max value out of the coupon. Thee are not transferable so you need to use it yourself if you get one. These are legitimate ebay coupons direct through the ebay site. It even displays my potential discount price under other items that I look at.

So if you are thinking of buying 2 things, make sure to buy the cheaper one first and see if you then get a coupon which you can use on the second item.

[reduno]

You may also want to sign up for the "eBay Bucks" program. This will automatically give you 2% back (in most categories) and they sometimes run special offers with double (4%) or even more back.