First thing you should have done is check out the "online service" that you used. Phone them first and ask what happens if you do not recieve goods as you stated. Write to their address - confirm they recieved your enquiry by asking them the address on the letter you sent.
If the "online service" company says they will re-imburse for losses you can go ahead and purchase.
If not, you should do the same as above with the person you are buying from. Get his name and phone number. Ask for his parents. Get their address (and write as above). Get I.D - passport details - offer a copy of yours. Do these checks and you are less likely to get scammed. You could also ask paypal if there have been any problems with that account, and whether they have the contact details of the scammer, and find out the level of their liability.
These are just basic security steps. It goes in the face of quick online purchases, but if the source is not trusted, or if they do not have phone or address
pr passport validation, then don't bother with them. If you have this, at least you can report the information you have to their local authorities. You must get the FULL NAME first, before you ask for identification, so it matches - they won't be expecting that and the will "run" if they are liars.