Hi, nancy! I assume you're talking about M&P soap exclusively? I wouldn't put unwrapped M&P soap directly in a cello bag, either - it will get the bag all smeared and gummy. I have used cello bags, but I shrink wrap the soap in plastic first before putting it in the bag.
I guess a lot of this is personal preference, but I usually wrap my M&P soaps in plastic wrap (like Glad wrap or cling wrap) and then use a heat gun to shrink wrap it. I liked this especially for craft shows because the plastic protects the soap and people can still smell the soap through the plastic. It also made set-up easier and tidier, too, because I could stack the soaps and people could clearly see what was on my table.
Also at craft shows, I used white paper lunch bags for customers' purchases. (I think the white looks nicer than the brown, and you can find paper lunch bags in a bunch of colors. I got my bags at Target and Wal-Mart.) You can stamp your paper bags or put a sticker on it with your logo and contact info, or whatever you want to dress them up.
Organza bags are also nice for packaging, but that can get expensive. Cello bags are more economical, and a bit of ribbon or raffia to tie off the bag really makes it pretty. You can buy cello bags in bulk online, but if you just need a few you can pick some up at someplace like AC Moore or Michael's in the baking/candy-making section for a reasonable price. Buying organza bags at a craft store would be pricey, so if you go with organza bags, I'd buy them in bulk at someplace online like Nashville Wraps to get a much better price.
When I make M&P soap, I let it sit overnight before unmolding, and I wrap mine in plastic right away after unmolding. It is important to wrap M&P to keep it from sweating and drying out.
Anne-Marie did a SoapQueen TV video about packaging here:
http://www.youtube.com/user/soapqueentv ... D4bJgtaHMgHope that helps! Good luck at your show!