The vast majority of people will not spend the sort of time you're talking about to select any piece of art. In fact, if a person leaves your tent without a purchase physically in hand it's very unlikely they will ever buy. I'd guess only a few out of hundreds will even bother ordering a size or frame that is not available on the spot to take with them. Some people do come back year after year to the same shows to see the same artists. They talk and ask questions, and some buy repeatedly. That's the extent of any "personal relationship" you're likely to develop. The vast majority of sales are made on the spot to people you'll never see again.
Also, you can't afford to spend hours on one sale. It costs thousands for a tent, display panels, print racks, and inventory. On top of that booth and jury fees at good venues are not cheap. It's not unusual for the fees to amount to $25 or more per hour that the show is open for sales. You also have to pay for the time and materials invested in the items you sell, some sort of basic wage for the hours spent setting up, selling, and tearing down, plus a reasonable profit. In short, you have to sell quite a bit to make it worthwhile. The larger your on-hand inventory the more you'll sell. Gallery sales are a little different, but even there people do not buy what is not physically available, and most sales are made in a single visit.