There is a small box to check. To see the exact instructions and where that box is, scroll down to Redeeming eBay Bucks:
http://pages.ebay.com/rewards/faq.html
They definitely expire, so use 'em or lose 'em.
Ebates is different - that rebate is sent right to your Paypal as cash and therefore never expires. Sports memorabilia and cards on eBay have been at 5% cash back for about a week now (usually 1%) -- if you are interested in signing up send me a pm and I will send you instructions and show you the ropes (I also get a bonus if you start shopping using Ebates). The % they give you is in addition to what eBay offers, so right now cards listed in the right category on eBay will net you 15% back if you use both - not too shabby.