How long ago was the purchase and payment made? And was it a credit card-funded purchase (or backed by something like a Visa debit card), or were the funds debited directly from your bank account balance?
If it was recently and the payment is being processed as an eCheque, it will usually take about 5 business days for the payment to clear. It's strange for the transaction to not be showing in your PP account though, so that can suggest something else is amiss.
Has the seller provided any info about the transaction? There are two different ways sellers can receive money now - either they get a PayPal payment directly to their account (in which case it will be very easy for a seller to verify and confirm receipt of a payment), or it goes to eBay and eBay then pays the seller afterwards. The latter system is quite new and there have been some teething problems (I'm on this newer system, and I've also had a couple of buyers message me about orders that never came through on my side at all, despite their cards / accounts being debited).
