So here is the deal.
Let's say you are cruising craigslist and you spot something you want to purchase. For the sake of argument, let's say it's an animal- a rooster.
First off, check the ad. Most ads are going to have a price and a location of the animal/item you want to purchase. Don't know where a place is? Google it. Use mapquest. Call a friend that is smarter than you are. There are options people. Decide if you are willing to go to said place before you contact the person selling the rooster. When I inquire on something, I know for a fact I'm willing to go get it. I don't care if you didn't know where the city was. That's not my problem. I don't care if you have to drive 30 minutes to come get it- again, not my problem. I state where my item or animal is, it's up to you to know where it is. Don't email me asking where it is either- because frankly, you could have googled the location in the same amount of time it took you to email me.
Make sure you know what a rooster is. Or what the breed is. Or whatever it is you are buying. Don't waste a few hours of the person's time then go "oh...you mean this is a chicken?" Again- google is your friend, if you don't know what a bantam cochin is- or a mini lop. Or a maltese. Whatever it is, google has it.
Next up, make sure you can meet someone when you set up a pick-up time. This waiting until a few hours beforehand to cancel or reschedule doesn't cut it. I know that things come up, we've all been there- but it is still very frustrating to someone who has probably already been holding an animal for you.
Don't save haggling for the end. Ask your questions about the animal or item, THEN ask for a price break. Don't get everything set up, down to the meeting time, then suddenly decide you really want to haggle an extra dollar off the price- again, you're just wasting time if you aren't willing to pay the price in the add. I'm not saying don't try to haggle, but again- either state your desire to get a lower price up front, or don't email.
Show up. If you have buyer's remorse and have decided you don't want to purchase your item, let the person know. Don't have them waste their time sitting around waiting on your, or their gas driving to meet you when you know you aren't going to be there. There is a special place in Hell for you people who do this.
Remember- be courteous and treat people the way you want to be treated. That's really all it takes.