ask the seller if you can see the purchase receipt or if he still has the original box.
As a seller, there are a bunch of red flags too..
One time I was trying to sell my MBP on craigslist. Had a guy call, he came over to my apartment complex and wanted to take my laptop real quick to "show his gf the laptop" to see if she liked it. Obviously I said no, she can come here and look at it.
Then he for some reason agreed to the deal and needed to get money. So I told him lets meet at starbucks around 7. He wouldn't come inside, wanted to stay in the parking lot in his car. SO I asked him to show me the money first, and had a friend of mine hold on to my laptop the entire time.
the buyer said "the envelope is sealed, it just came straight from the bank"
RED FLAG. No banks are open after 5-6pm...and chances are was that envelope was full of $1 bills. Called him out on his lie, and told him to GTFO before I report him to the police.