When I was younger, in my 20's, working out of a pickup and knocking on doors I used to use the old "if I can do it right now" line and would often get the job. I don't do that anymore because I think it make one sound too much like Vince from Shamwow.
I do understand that your not saying that you used that line, rather you gave the guy a price that you thought no one could refuse and he balked on you. I have made the same mistake before. Just chalk it up to a lesson learned. I will use the do it now price just like the other guy mentioned. If I am working next door and can do the job while I'm there without having to travel to return I will tell them that I will do it right now for this price but if I have to return to do it it will be this price. I will also often ask a potential client, if I am not busy that day, if I bring my climbing gear and we agree on a price that day if they would like for me go ahead and do it. It allows me to gauge the person and see how serious they are about getting the job done or if they are getting competitive bids.
Just a little food for thought:
There is a guy working in my town that I have had two clients accept my bid over his even though I was the higher bidder. I have never met the guy and don't know what his personality is but I do know he is loosing higher end clients because he is under bidding. One of my clients asked me to come put a sign in their yard so the guy would quit bugging them. LOL, after I did the job he stole the sign out of the clients yard and they went through the roof and asked me to bring another one and put it in there yard. My other client asked me if I knew the guy and what I thought of him after she had awarded me the job. I told her that I hadn't seen his work but related the story of client #1 with her. She told me that he had bid $300 less than me but just didn't have a good feeling about the guy. Not everyone is necessarily looking for the cheapest price when considering a tree service. A lot of them have had work done before and know what it costs. If they think that you don't know what it's worth then what kind of service are you going to provide them? Just something to think about.