Below market value, asking for cash before giving anything of value in return, no documentation and disclosure provided, lack of transparency - all of these are suggesting that you are dealing with a scam.
I see other people answered right away too. This is a common scam found on Craigslist. No owner or broker would ever ask you for that. Remember if something seems to be too good to be true, then it probably is!
Never, ever hand money over unless you are applying for the apartment or signing leases. The only other time you would hand over funds is while the application is being processed and you are asked to hand over a "refundable" good faith payment.
If you would like some help in your apartment search please feel free to give me a call.
Bond New York
Licensed Real Estate Salesperson