I'm certain that others have more experience than me, but this seems right at the very edge of credibility to me.
I have two nursing homes that are my best accounts, they each do $200-$300/week out of a 4 wide snack and a DN5591. After seven days the drinks are pretty decimated and the snacks at least half empty. All the locations you are talking about would appear to just have two machines. My point is, depending on the equipment, to get to most of those numbers, he would have to be servicing them twice a week. Is that what he is telling you? I've never seen a vending business for sale in my area with such a collection of great accounts and no duds - even the office locations are doing good business which rarely happens. And all for sale for 6 months gross?
If its accurate, its a tremendous deal and will be sold quickly. He's gotta have some sort of documentation he can show you. Deposit slips, receipts for inventory purchases, etc. That's a lot of cash, there must be some records of it being deposited or spent somewhere.