I think a lot of it has to do with how you go about your business. Most campground managers are pretty darn busy and just wishing for no complaints and happy campers.
Casually handing out business cards to people you meet is pretty common and acceptable. Placing stacks of them around without permission is likely to be frowned upon quickly. I've seen plenty of magnet signs and painted signs. A lot of constructions folks fulltime in campers, so they often have work trucks with their business painted the truck. I've seen folks put up small signs in the windshield of their rig. One guy claimed he did it to write off his RV expenses but wasn't really working (but he seemed real popular!)
A friend of mine asked permission in a friendly campground to have a small rummage sale off the back of his pickup truck in the mornings. He was moving overseas and emptying out a large 5th wheeler. He only put out about ten items (on the end of his truck bed) each morning for about an hour or two. The next morning, a different collection appeared. He sat outside with his coffee and closed up "shop" by 10am. People loved this idea, walkers passed by his truck every morning and he quietly without a big mess or ruckus, emptied out most of his 5th wheel over the course of a few weeks. Even the campground manager kept stopping by to see his latest.
I've seen many artists and crafters, do their work outside, casually scattering their finished products around their campsite hoping for "inquiries" without hanging up a shingle or making a mess. Most cleared up their stuff when they weren't outside working on it.
One enterprising couple had discreet small paper tags hanging off their beer can whirligigs which were attached to their awning. So it looked like art in motion, but the stringed tags suggested they might be for sale if one asked.
I've seen folks prop up an item for sale, and when it sold, out came another one, just like it. Such as the guy that was selling this laundry hanger that hung off the back ladder and allowed you to hang clothing hangers from it to dry things without the hangers being able to blow away or fall off. As soon as he sold one hanging off his ladder, another one magically appeared.
In another campground, a lady paraded her shaggy doggy around, then she set up a table in front of her camper and spent a long time grooming her dog. Another parade around the campground of the groomed doggy and soon it seemed she was busy grooming dogs for free (but accepting tips).
I saw a lady publicly and expertly cut her husband's hair at their campsite. When walking her dog she told folks she was a retired hair dresser. She trimmed up quite a few heads here and there as a "favor" but it seems *ahem* her happy hour cabinet was becoming well stocked in short order.