Could be a bunch of reasons. The three most likely are:
1. You didn't have enough PayPal balance to cover the purchase, so money was taken from your linked bank account to cover the payment. Until the withdrawal from your bank account is confirmed/cleared the payment will be on hold.
2. Glitch on PayPal or eBay's end. Ask the seller to log into their PayPal account and check to see if the payment arrived. It could be sitting in their account and they just don't realize it.
3. If they are going to log into their account to check for the payment also make sure that their PayPal account isn't set up in a way that they have to manually accept each payment. If it is, they just need to log in to their PayPal account and accept the pending payment.
My guess is that they are doing everything through the eBay site, but if they log into their PayPal account it will probably clearly tell them why the payment is pending.