Because it is a unique 1/1 and was signed by the original initiator of the Austria Post Stefan Nemeth back in 2019.
It is definitely worth more than a regular set.
unique 1/1 - what is so unique about it? every code is unique, nothing special about these stamps. codes themselves are -as far as they are readable- not special (neither "only lowercases" nor "only capital letters")
signature of a -sorry to say- more or less no-name also does not justify such asking price imho
but time will tell and I hope I'm completely wrong on this. I wish you good luck with your sale!
(you're the Tyrolean guy, aren't you?)