After being offered a nine week old spaniel with a Hungarian vaccination on the internet site willhaben.at, Four Paws campaigner Indra Kley suspected the seller was not an approved animal dealer.
The City of Vienna sent an undercover purchaser to investigate. The puppy was confiscated and subsequently taken to an animal shelter in Vösendorf.
"The fact that the dealer knew their business was illegal was apparent because of numerous inconsistencies... and the photo in the ad did not match up with the actual puppy," said Kley.
The price also increased from the advertised €179 to €280 and while the seller initially told the undercover purchaser the puppy's mother was Austrian, she later said it was Hungarian.
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According to the Austrian Society for the Prevention of Cruelty to Animals, more than 100,000 young dogs are transported annually through Vienna. Often they are ill or very young.
Animal welfare councillor Ulli Sima called for people to refrain from purchasing pets over the internet.