DO - Make sure you have heaps of plastic bags handy, as its certain that customers will ask whether you have anything to carry the items they have just purchased. Grocery bags are ideal for this.
DO - Have an extension cord ready to prove electrical items work. Even better, with items such as TV's, Fridges, etc, leave them on so cusotmers can judge their quality and conidtion without having to ask you all the time.
DO - Have lots of change ready. Most customers will not have the exact change when paying - in fact, they'll likely pull out a $20 or $50 dollar note, which can be quite irritating (now you understand why Cab drivers get peeved when you pull out a $50 note for a $8 fare).
DO - GUARD YOUR MONEY! Keep your change in a bum bag or somewhere secure, but make sure its within reach to give customers their change.
DO - To avoid any hassles later on, post a sign that says "All Sales Final". You don't want customers returning the next day wanting to get their money back on items
DO - Have a contingency plan for wet weather. Consider putting up tarps or holding your sale under cover.
DONT - The first person up your driveway will probably try to negotiate, but don't drop prices right away. If someone makes an offer, write down their phone number. (However, for items of smaller value I wouldn't recommend such a ploy - its false economy)
DONT - Don't let your pets run loose. Animals are an unknown factor and will most liley cause the unexpected to happen. Instead, keep them out of the way. Remember, in these days of public liabilty, any mishap or incident is a potential court case waiting to happen.
DONT - Don't accept checks unless you are willing to take the risk of getting a bad check. A check that looks perfectly fine may be from a closed bank account.
DONT - Don't ever lie. Always give an open and honest opinion about what you're selling. If something's broken, faulty, tarnished and simply in need of some TLC, then say so upfront and leave the potential arguments and disagreements for someone else.