This is why good communication is key when making a deal. Although there are times shit does happen and you need to back out of a deal, the person who is trying to buy your items should communicate this. We all know and understand this can happen, but to lead you around on a leash for so long then drop out last minute is just not good form.
On that note, communication is required to establish intent. If I'm just asking questions and only half ass thinking about buying an item I will let he buyer know that I'm "interested only" or to leave the item avliable to anyone else who has the money. On the other hand if I know with 100% certainty that I want that item I'll let the seller know what my intentions are, down payment, payment plan, when I get paid, etc. Work out all the details of what and how the deal will happen. If I'm just asking questions about an item I will state this. But no matter what I always do my best to make an active effort to make sure the person on the other side of the deal knows what I am doing, why I'm asking questions, or if I'm really serious and need to make some sort of special arangements such as waiting to send money until payday.
It is always a good idea for you to establish, espeially with a big ticket item, that if the person your trying to sell to has a low trader rating and no other forms of feedback, that you will be more specific with your communication to him outlining what you want out of the deal. The feedback rating could also be used as yoru own person guide as to how far you will trust that person with such things as payment plans, waiting until thier payday, shipping first before receiving payment etc. I know on some forums that people will refuse to deal with a person unless they have a feedback of at least 10 or more. Other times I have delt with people with zero feedback and the transaction was perfect and no worries.
Now after my ranting here, in your case it's a matter of how you feel, but if you think that the person made you feel you had come to the conclusion of a deal then yes, I might have left neutral feedback. I'm not saying this is a hard and fast rule, but if you feel that way then you should not feel bad for leaving it. A person will only learn how to be a good trader on a forum by learning and that means they have to be taught.
If you feel you have good communication on your part, left the other person knowing how you would like to proceed (ask questions, get payment info), that you want to know a ya/nay answer on if he wishes to proceed with the transaction etc then yes nuetral feedback is needed. However, if you were just answering questions, with no clear and consice statement that you consider him to now be the future owener of said item, then I think it falls on your part to not leave feedback and to learn from the experiance to establish better communication in future deals. Don't forget, if you don't ask if he is 100% on the deal, then you never really know. If he doesn't say he's 100% into the deal then you don't know. This is what ultimatly has to be said by one of the people involved.