My local food coop was selling soap by the slice for a time. There were several loaves of soap in different colors and scents in a rack, a bench scraper to do the cutting, and a cutting board. They let customers do the slicing. Because the display wasn't in a well monitored area, there was a lot of potential for theft/loss, product contamination, and mess. I think it would be better for the staff to do the cutting, as you are thinking of doing. I didn't cut or handle any of this soap myself, but it didn't look like the soap was overly hard to cut judging from the cut surfaces that I looked at. It appeared to be CP soap, not HP, or possibly a melt and pour soap that had the look of CP soap -- the loaves had that opaque creamy look that CP soap has.
Once upon a time (before I learned to make soap), I was buying 3-pound loaves of HP soap from an online soap maker and cutting the bars myself. At the time I didn't know how old the loaves were, but I now suspect they were just a few days old, because they were soft and easy to cut like soft cheese. I tried a few bars right away just because any handmade soap is better than store-bought stuff, but I knew enough to lay most of them away to cure before use. Even though they were mild and skin safe, the freshly cut bars mushed up easily and did not last long in the shower. If I hadn't known the bars needed to cure, I would have been really unhappy with this soap.
At a show, if you're selling well, I agree with Shunt. It's going to be a challenge unless you have a helper to serve other customers while you're doing the cutting, or vice versa.