Not necessarily
believe it or not, there are people buying it and then decide to sell it again for whatever odd reasons.
We have an enormous book store specialized in 2nd hand copies, they have about 9 settlements divided over Belgium and the Netherlands. And they seem to have had a 2nd hand copy of this book in-store lately. But already sold again as stated.
Gezaghebbende uitgave over het complete aanbod aan aquariumplanten Aquariumplanten beschrijft alle gangbare aquariumplanten, in totaal maar liefst 45...
www.deslegte.com
They also sell books in German and English language as well, if you ever are in search of something sold out on the regular market. It's a chance they still might have a copy of it hidden in dust somewhere. Or you can subscribe to a mailing list to receive a notification on future availability.
Unfortunately, they do not have an international website.