It depends on the nature of your purchase, the funding source used and when you became aware of the problem.
For example, if you purchased concert tickets via eBay, then there's an undocumented extended buyer protection programme, which was introduced after Michael Jackson's death. To file a claim, you have to contact eBay customer support directly - within the 7-days following the concert date.
If you funded the PayPal payment with a debit or credit card, then your card provider may be able to recover your money with a chargeback. For Visa cards, you have to initiate the chargeback within 120-days of becoming aware of the problem (up to an absolute maximum of 540-days). In the case of goods not received, the 120-day time limit countdown, usually, starts from the expected delivery date, please see: Visa factsheet (pdf)
NB There's no minimum or maximum amount for a Visa chargeback, but a Mastercard claim has to be for at least £10.
