Yes, it's a no hassle add-on feature to your ebay account. Just keep in mind that these Ebay Bucks promos are sometimes targeted and not everyone will be eligible for the 8% but anyone can sign up for Ebay Bucks and get a certain % back (default is 1%). I'm not eligible for the one today for example but I was 2 weeks ago when they did a 8% back.
Basically, it's broken down to every 3 months they take the total amount spent and give you a % back in form of ebay credits which you then have the 4th month to redeem. E.g. Let's say I buy $200 worth of items in April, May and June ebay will then issues me say $X to spend by the end of July. I haven't bought much but the 4 items I have purchased since April 1st has earned me $0.63 so far which they will issue a certificate for in July. I'll be purchasing more things but I average around $10 each quarter since they've been doing less site-wide % off promos and more ebay buck promos.
They often do 8-10% back although I've seen 15-20% as well. I earned over $100 one time when I started buying bolt-ons for the SJ (handle bars, grips, ride plate, intake grate, pump parts, bilge kit, etc) during a 15% ebay buck promo. I used that $100 the next month to "buy" hydro-turf.