If there are no buyers - validation done.
If there are buyers - refund money, apologize for delay, send X% OFF coupon and start building it.
Maybe a bit controversial but works with many types of products or services.
(About 15 yrs ago I had an idea to combine all esoteric spiritual ideas that belongs to certain fragmented discipline into one place and sell it as eBook. That exactly what i did and was very surprised to give 4 refunds within short period of time, before putting it "off market")
Another alternative: put up flyers. When someone calls, say that the service is currently fully occupied. (works great for car rentals, cleaning services, etc)
After validating, you might want to shut down the advertising.