I have seen a number of vendors selling "Bitcoin private keys". For those fairly new to the scene or unaware of how Bitcoin works, I just wanted to implore you to NOT BUY THESE.
They are not selling keys to addresses that have balances, ever. Anyone saying that they found coins in any of these packages is lying, they are sock puppets of the shady vendors.
All they do is generate batches of new Bitcoin addresses using software like Electrum. This software is free, and you can generate new addresses yourself just like they do, only it will be FASTER THAN IMPORTING THE ONE YOU BUY!!!
The thing is, you could do this for the rest of your life and you will never find a dime. It is hard for humans to get their heads around large numbers, but you could import 100 million of these private keys every day for ten years and odds are you would never find an addresses that has ever been used, let alone one with a balance. This is why Bitcoin works. If someone could just generate a million keys and find other peoples' used addresses, the entire thing wouldn't work.
I am not going to shit on any specific vendors... they might not understand this fact themselves for all I know. However, if they are honest they should really stop selling these. Anyone claiming that anyone found a balance with any of these, or used a sock puppet account to claim that, is lying and should be considered a stammer and banned IMO.
TL;DR- There is zero point in buying Bitcoin private keys. It is a waste of money, you will never find a single Satoshi. Plus you can generate them faster than importing them using Electrum.
I'd imagine the really crafty vendors would keep a copy of the sold private keys and write a bot to sweep any balance that comes into the address.