Great thread! I only do outdoor markets/festivals so maybe I can provide some tips.
As others have said, weights for your tent are a must, and most outdoor venues require them. Not only can you lose your tent, but you could hurt people if it blows away. Most commercial tents come with plates that fit on the bottom of the the tent legs (upon which you place something heavy - most vendors I know use cinder blocks), but IMHO these are an eyesore and far too easy for customers to trip over. I use 50 lb weights on each corner which are anchored to the TOP corners of my tent by parachute cord. These weights are 2 ft long sections of wide PVC pipe filled with concrete with heavy-duty 8" eyelet screws embedded in the concrete at the top to attach the cords. My husband made all four of them for less than $20 - and short of a hurricane/tornado, my tent isn't going anywhere!
My tent came with sides that you can attach and zip-up/close which are very handy for multi-day events. So I COULD use them for shade, and/or rain but they aren't very pretty. When it rains, I cover my tables with clear painter's dropcloth from the dollar store. When the sun is blazing and my products and I need some shade I use pretty fabric that I've acquired from Walmart and fabric stores for less than a dollar a yard - I buy this stuff by the bolt. Just measure it out beforehand and apply velcro to fit with the velcro in the sides of your tent.
Money - If someone is trying to buy something less than $20 with a $100 bill, I politely direct them to the person in charge of the venue (as they usually have change for big bills) and also let them know that I take credit/debit cards via Square. Sometimes wifi is sketchy at outdoor venues, so I usually keep my iphone set as a 'hotspot'. I've discovered that this alone can make you some serious friends among other vendors
As someone else said, I rarely sit down when doing markets and festivals but I try to unintrusive and let them shop in peace. I just want to be accessible and interested in my customers - IMHO sitting on a chair playing on my phone doesn't convey that!!
I do however keep an extra chair at the corner of my tent for the people who just need to sit down for a minute. Established markets usually have benches thoughtfully placed for the customers, but not so much at festivals.
I also keep a couple of gallon jugs of distilled water in my cooler and paper cups under my table at festivals. Not that I advocate breaking the rules, but I'm not comfortable with letting someone get dehydrated in 100 degree temps because they didn't want to pay $$$ for a bottle of water!