There is no buyer. There is only a scammer who stole your camera lens. Now the scammer is trying to steal your cash via Western Union. Cease emailing that criminal immediately.
The next email was from another of the scammer's fake names and free email addresses pretending to be "Paypal" saying "kindly send the tracking number and we will release the funds". The next fake Paypal email will insist you send cash via Western Union.
Paypal does NOT send such emails, ever. Paypal does NOT have escrow or money holding services like that scammer describes. Paypal does NOT demand you send a tracking number before money is sent. EVER. No exceptions.
Western Union and moneygram do not verify anything on the form the sender fills out, not the name, not the street address, not the country, not even the gender of the receiver, it all means absolutely nothing. The clerk will not bother to check ID and will simply hand off your cash to whomever walks in the door with the MTCN# and question/answer. Neither company will tell the sender who picked up the cash, at what store location or even in what country your money walked out the door. Neither company has any kind of refund policy, money sent is money gone forever.
Now that you have responded to a scammer, you are on his 'potential sucker' list, he will try again to separate you from your cash. He will send you more emails from his other free email addresses using another of his fake names with all kinds of stories of being the perfect buyer, great jobs, lottery winnings, millions in the bank and desperate, lonely, sexy singles. He will sell your email address to all his scamming buddies who will also send you dozens of fake emails all with the exact same goal, you sending them your cash via Western Union or moneygram.
You could post up the email address and the emails themselves that the scammer is using, it will help make your post more googlable for other suspicious potential victims to find when looking for information.
Do you know how to check the header of a received email? If not, you could google for information. Being able to read the header to determine the geographic location an email originated from will help you weed out the most obvious scams and scammers. Then delete and block that scammer. Don't bother to tell him that you know he is a scammer, it isn't worth your effort. He has one job in life, convincing victims to send him their hard-earned cash.
Whenever suspicious or just plain curious, google everything, website addresses, names used, companies mentioned, phone numbers given, all email addresses, even sentences from the emails as you might be unpleasantly surprised at what you find already posted online. You can also post/ask here and every scam-warner-anti-fraud-busting site you can find before taking a chance and losing money to a scammer.
If you google "cragislist buyer scam", "fake paypal email scam", "ebay escrow fraud" or something similar you will find hundreds of posts from victims and near victims of this type of scam.
Check out the one and only official paypal website, read up on what paypal does and how it really works.