How bout' you send me 5% of one of those wallets as a sign of good faith and in return, I will buy the private key for 15% percent of the balance of that same wallet. Or send me 5% of a lower balance wallet and I will in return buy the wallet a grade up. How's that sounding for you? Also, there is a Prince in Africa I would like to introduce to you, he would send you 1 bitcoin but you have to send him only 200$ as a show of good faith. SEEMS LEGIT!
If it was ok for me to do a transaction and send you 5% of the wallet, why would I do that instead of cashing out 100% of it for my self?

What a nice joke that Prince

I see it doesn't make sense to you as you don't have a clear understanding how this works. Say by some means you managed to get access to lots of private keys, not one or two, not even a dozen, lots of. And of course it's not the last day of your life on earth. You want to make money, and use it so you don't want to get cough, what would you do? How would you cash them out?