Just to make it clear - automated scripts of this nature are prohibited by GAW and if someone indeed used some kind of scraper like that to buy OP's hashlets the transaction should be reversed. However there isn't any way to prove it as far as I know.

FWIW this is far from an isolated incident. The Zen marketplace user interface is notoriously clunky and unnecessarily confusing. For example the cloud's own price list shows hashlet prices per MH/s and there is no reason why users selling on the marketplace should be required to enter the TOTAL price. If the price was entered per MH/s the risk of incidents like this would be greatly diminished. I'm not saying it's not the OP's fault, because sadly it is, but there are some very simple tweaks GAW/Zen could and should implement to prevent this from happening.