How did you pay for these tickets? Unless you paid via PayPal, I can't see a solution that isn't going to cost you any money.
If you have paid via PayPal, you need to report the seller to them immediately. As a precaution, they freeze any funds in his account and communicate with both you and the seller to resolve the issue.
I think when you are purchasing something like this, you should always request that goods are sent via recorded delivery. But why not ask the seller for his proof of postage? Whenever somebody posts a parcel, the receipt given to them will act as proof of sending.