Could you tell from the message whether it was the payment that was declined, or the transaction itself? If you log into you PayPal account and there is still a record of the transaction in your history, it might be able to provide a bit more info.
Some, though usually not many, sellers have an automatic block that prevents buyers from purchasing above a certain number of items in a 7 day period, though it would be strange to have a limit of 2 for low value items (especially if people often buy several).
Other than that, it would have to be a PayPal or (possible) funding source issue (eg credit card declined - which could be for any number of reasons, if the bank or PayPal decided three identical, consecutive transactions looked sus). As above, though, since it's only speculation, the best thing to do is call PayPal.
