You can not venmo money instantly. The UI makes it seem that way, but it takes a few days for payment to process and as such, Venmo has become a target for scams: http://time.com/money/4036511/venmo-more-check-than-cash/

So do not use Venmo for commercial transactions (buying things of Craiglist for example) and do not assume the money transfers immediately.

Also shame on Venmo for misleading their users; they should do a better job of explaining how the transfer actually works.