Baked goods, drinks, sandwiches, etc. are good for the kids to sell. Cover up any items not for sale. Have change-coins and dollars. Always keep an eye on your money. Remove large bills frequently.
Large quantities attract more attention.
For multi-family sales, have all items tagged with prices and initials. Have a notebook with several pages for each family to keep a tally of the sales. Decide ahead of time if you or your neighbor are negotiable on prices. For large items, record the lowest acceptable price in the back of your notebook. After the sale, be sure to remove your items and close the garage doors.