@Lewis B
I have been on both sides of the table. As a seller, my teenage, technical smart teenage daughters set me up with ways of taking electronic payments. I personally accept Paypal, Venmo, Cashapp, and Apple pay. With Paypal nad Venmo, I have sold items as low as $20 and as high as $5500. If the seller has either of these applications, their credit card information is already loaded with Paypal, Venmo, etc. and can be charged instantly.
We agree on a price and the buyer scans my QR code at the table.
They type in the amount to pay, and send.
About 1-2 minutes I get an email showing $$$ has been received into your account.
As for shipping, I have never accepted money at the show, then shipped afterwards. It is not that I would not do this for someone, but it has not happened for me in this way. I would have no problem returning home, and carefully packaging the item(s) and shipping to the new owner. <-- If I was a buyer, I would make sure to get a business card and full name of the seller. If something should go wrong, you could always turn to the person who runs the show to provide additional information on the person who you are dealing with.
Wishing you all the best in your collecting journey!
Dan