I'm not entirely sure why but this seems a bit unethical to me. It is quite clever but maybe you should make it more clear that you are doing a survey and not actually selling your book.
The only valid signal for willingness to pay is ACTUAL WILLINGNESS TO PAY. Ideally, you'd even collect (and discard) a credit card number. Asking people to take a survey gives inaccurate results
At that point why not just sell the thing? The per unit cost of an ebook is under a penny, who cares if it sold for $6 or $9.. if they pay, give them the download.
There were a few reasons why I chose to say 'This product is not yet available yet' rather than sell it. First, I was going to do a full product launch and didn't want to release the product into the wild yet. Second, as comments stated below, if one person bought it at $100 and realized someone else bought it at $5, they would be angry. They would ask for their money back. If I were going to eventually charge $50 for the product (which I did), the people who bought it for more would be pissed.
You could give it to all your first customers for the lowest point offered. That is, they said they would pay $60 but you only charge them $20. Tell them why. Tell them this is basically an introductory price and it will go up, but because they helped you they are special. Of course, this limits the N you get for your testing, but it gets rid of the ickiness of it all.
It seems unethical because it is. Making what appears to be a genuine offer to sell at a given price then saying "haha, sorry, just wasting your time to help me but without paying you for it, I'm not selling" is unequivocally a dick move.
