eBay periodically offers site-wide discounts, and today the site is bringing it back again. From now until 8PM Pacific Time/11PM Eastern Time, you can save $15 on almost any purchase of $75 or more.
There are a few minor catches; gift cards and coupons are not eligible, and your total has to be greater than $75 excluding shipping, handling, and taxes. You also can't use this discount on eBay Real Estate - $15 off a new house probably wouldn't do much good anyways.
All you have to do is use the coupon code PREMEMDAY15 at checkout. Pretty easy.