I've sold in a few Mom2Mom sales through my twins mom's club, and also organized several of our sales. I always price my items at 1/3 to 1/2 of the original price I paid for the item. I always take in to considering the item's condition, and if I bought it second hand. I also decide how badly I want to get rid of it too. Clothes that I had bought second-hand but were in really good shape, I usually priced lower than clothing I bought retail. Toys, books, and puzzles are sometimes harder to sell, but it depends on your area and what people are looking for. With toys and equipment, I always made sure it was clean, all pieces/parts were included, and I would print off the MFG info on the item (along with the retail price -- use a site like Babies R Us or the MFG web site of the item) so that people would know what ages the toy was appropriate for, or for example, in the instance of a stroller, what the weight limits were, etc.
I also would group items together and sell as a package. So for example, instead of selling a stroller, and stroller rain cover separate, I put them together and sold them together. If I had a couple of Curious George books I would bundle all three together. Onesies I would pin three together with a large safety pin and sell three together for 1 price. Same with Socks -- I would put socks in a basket and a sign saying 8 pair for $1 (or something similar).
The better you organize your table and make it easy to find things, the more people who will stop and look and buy. Find out from the organizer if you can bring along plastic stackable bins or shelves to put in front of your table (sort of underneath) because you can put books and toys on these, shoes, etc. Also, if you can have a clothing rack, use one, it's easier for people to browse clothing when it's hung up. Sort it by Size. Take a piece of poster board and hang it on the end of the rack and write down what sizes of clothing you have (i.e. Boys 2T - 4T, Girls 3T - 5). If you are selling both boys & Girls clothing, keeps the boys clothing on one side, girls on the other. Be ready to deal and negotiate.
Things that did not sell I would sell at a consignment/resale store (Children's Orchard is a good one), sell via Craig's List (for larger items and equipment), and most clothing I just donated (honestly, I can get better prices for tax-write offs by donating clothing than what I can sell most of it for. I live in an area where there are a LOT of Mom2Mom sales, so you can't get very much for second-hand clothing).
--K.