There are a few possibilities.
Did you use an unconfirmed shipping address?
If so, he may have his PayPal set up so that he has to either accept or reject PayPal payments from buyers with unconfirmed shipping addresses. I have had this happen a few times because I like to have my chips shipped to my PO box which is unconfirmed (not the address my bank statement goes to) rather than my home address. It has never been a problem, but sometimes it takes a few days before the seller accepts the payment. If they were to insist on sending it to a confirmed address I would just switch it to the home address.

Another possibility is that you sent a payment funded by Credit card and they don't accept credit card payments. Many people are more than willing to accept PayPal payments as they are the most convenient for both buyer and seller, but they only have a "personal" account rather than a commercial account. This means if someone tries to send them a payment funded by credit card the payment shows as pending and PayPal tries to get them to "upgrade" their account in order to accept this payment. I agree with Pete.. the problem with this is that if one accepts this payment and upgrades the account all subsequent transactions are subject to PayPal fees of 30 cents per transaction plus the 2.9% fee. This is part of the reason you are seeing many sellers charge higher shipping and handling fees. I personally try to keep shipping fees reasonable by not upgrading to the business account but I still accept non-credit card PayPal payments for my customers' convenience. Since most of the items I sell are below $10 and I only charge $1 for combined shipping, even to Europe and Canada, I had to decline the PayPal "upgrade" (downgrade)

The third possibility is that you somehow got a typo in the PayPal email address of the seller. Did you click on the PayPal button in the auction to make payment? Go to your PayPal account and click on the details of the transaction and compare the email address to the one on the button in the auction. You can always cancel a transaction if it has not been accepted yet.