He seems to be upto something! The 3 coins he sold to you maybe a "bait". Why wouldn't he not auction the coins online if he needed money? He isn't expecting someone will handover 150K in cash?
Yes, something is smelling fishy here. A lot of people use situations like this to exploit and scam people. They gain your trust by selling you cheap coins at bargain prices and once trust is gained, they encourage you to buy in bigger volumes. Once a big sale goes through they find a way to run with the money, before they hand over the coins. If this is a cash purchase, the thugs wait for you and they rob you. So you should always do this in a public place, where there are CCTV cameras and many law enforcement authorities around.
People are very creative and before you know it, you are robbed and out of pocket with thousands of dollars. If it is too good to be true, it most probably is.
These people might even have stolen these coins and are off-loading the stolen coins onto you. You will have zero evidence that you bought it off a guy from the street. ^hmmmmm^