Don't use Venmo at all. I've only used paypal. Looking up Venmo's security policies I see on their website:

"Keep yourself safe
Venmo is designed for payments between friends and people who trust each other. Avoid payments to people you don't know, especially if it involves a sale for goods and services (like event tickets and Craigslist items). These payments are potentially high risk, and you could lose your money without getting what you paid for. Venmo does not offer buyer or seller protection. Business usage of Venmo requires an application and explicit authorization. For more information, see our User Agreement."

They offer zero protection. Looks like a bad version of F&F from Paypal.

I would have refused or suggested a different service. Thankfully, it was only $100, but still, not fun to have to deal with. Like others have said, get started with a machine locally first. Cash on the glass.