I've been selling records on the side at record shows for nearly a decade. I started with a couple boxes, a couple crates and a box for 45's and now take two, eight foot tables worth (1K+ LP's & 45's) to shows I can reasonable drive to and home from same day. I do between 4-6 shows a year all over the Gulf South.
Here are some tips:
Just get small Uhaul boxes. They're a $1 each, they stack well, and carry less awkwardly than anything else I've hauled. After a few uses, they do get worn down. But then you can easily and cheaply replace.
Just get your own Square reader. If there's not enough time, borrow one. But Square will give them to you for free just using their service. Check their fees, but it's reasonable. I think when I did $840 in just cards one time, it worked the fees out to around $20. Or get some comparable reader like Paypal. But make a sign that you accept credit cards. Even at festivals, people forget just how readily unavailable ATM's are. If you're one of the vendors who takes credit cards, you'll sell more. I promise. I resisted for so long but it just makes sense. People usually want to buy, so give them a reason to charge something.
I divide my boxes by genre. I don't have enough enough space or records of one genre to really get sub genre specific. So, there's a couple punk/metal/ska/indie boxes. There's a jazz box. There's five rock/pop/folk/country/electronic/rap boxes. There's a couple soul/blues/gospel/reggae boxes. There's a soundtrack/spoken word/comedy box. I obsessively alphabetize to help people (and myself) locate things easily. Plus, I know immediately if I have something or not because I've wiped, cleaned, bagged, listened to, researched and priced every goddamn record I have. It's easier than you think to keep track of stuff. There's maybe two records in my nearly ten years that passed through my hands that I'm not sure if I sold or they got stolen.
As you get addicted to this, buy a dolly. You can stack three of those Uhaul boxes up safely to load in and out. Saves on the back. I live on the second floor of my apartment. I dread making fifteen trips down to load and vice versa.
Just buy little white labels at Wal-Mart. Stuff that needs to be bagged you can write a blurb and price. If it's cheap, don't worry that the price tag is on the sleeve.
I always try to set up closest to the entrance. I want to be the first records people are looking at. Sometimes this isn't possible. But if there is a first come, first pick table policy, I'd get there super early. When other vendors start breaking down tables, I keep mine up until the organizers say it's time to go. I've sold a ton sticking around until the very end.
Make up business cards. If you sell on discogs, just have that address, your email, other contact info on the card. People often want to buy records, so be their guy. Not everybody is as resourceful as we are. But people on occasion are also looking to sell collections sometimes. I've never actually gotten a call this way, but I need more records to buy like I need a hole in my head. If you want to do this more regularly, you got to let people know you're the guy who buys records. I literally just bought 1,100 LP's off Craig's List. If you advertise that you're buying, people will get into contact with you. It helps if you're one of the only people in town doing this. I have a real job and there's tons of people here. But sometimes you get lucky. This was in the collection I bought, which is part of the reason I forked over the dough:
Good luck! Any other questions, just ask.