Quoted from sbmania:

No need to continue this thread. There is only one way to pay when buying a game at a show - cash. Not checks, credit cards, Paypal, escrow accounts, promises, pinky swears, or anything else. If you want to buy a game from someone, bring CASH! The only exception might be a national level dealer who might take credit cards. But don't expect to negotiate the price any if you're not paying CASH.

Quoted from CrazyLevi:

Cash if you want to be sure you don't miss our on a deal.
I've bought several games through paypal or IOU at shows though. Guess it depends on if the seller likes your face.

Quoted from Grandnational007:

In all seriousness, why wouldn't Venmo work?
It's instant. It's un-reversible. It's free.
(not sure if there is a maximum amount to be sent/received, but I haven't hit that threshold yet buying and selling some bigger ticket items when cash was inconvenient.)

Quoted from Grandnational007:

You can't send direct to a routing number; rather, you send/receive to/from a verified venmo account, that MUST be linked to a bank account. Thusly, venmo is the in-between, and I'm imagining, makes money from people who don't immediately transfer their venmo balance to their bank account. (It takes 1 business day to transfer from your venmo account to your bank account for free (otherwise, its like a 3% fee to transfer instantly, hence my previous thought...)
I usually leave about $500 in my venmo account (even though transferring from your bank account to your venmo account is instant) just so I can snap up good deals when I see them in another hobby I'm into (where venmo is the preferred payment method). I'm guessing, aggregated over all the other venmo users who have money in their accounts, adds up to real interest money for them.

Quoted from sbmania:

If you go to a show expecting to buy something with venmo, you better hope the guy that has the game you want either has a venmo account or has interest in setting one up. Call me old fashioned, but I'm not interested. It's too complicated and just seems ripe for some kind of scam.
As far as Paypal, maybe from a personal friend, or someone extremely well respected in the pinball community, but aside from that, not for me. Too many scams. Also, every time in the past I have taken Paypal, there are daily limits on how much money you can take out at an ATM, and Paypal charges a couple of bucks each time you use the ATM. It's a pain in the butt as far as I'm concerned. When I sell a game, I want the cash then and there because I am most likely gonna spend it 10 minutes later on another game!