This is such a common scam unfortunately. Do not hand over any money until you are at least sure the animal even exists! Always assess the home conditions and health. I would never buy a dog from an unrepetable breeder ever again. It can cost you hunks of money in the long run. Not to mention not knowing where it came from! I have had bad luck...
I bought a "cavapoo" from Gumtree many years ago, the designer mutts didn't cost no where near as they do now back then, I had the budget of around £300 and set about looking for a small, cross breed on Gumtree. The Cavapoo stuck out to me, they appeared to be double even triple the price in other ads. She was advertised as only "one in a litter" it was a lady from the like, who said there is no way I wouldn't fall in love with this puppy and it would suit her better if her husband would bring her to me. He did within an hour, no money was exchanged until he bought this puppy out of his coat in the palm of his hand. I was eager to ask lots about her, what she had been fed, etc. But he grunted at me and turned on his heels as soon as the money left my hands. He seemed so unsavory. I noticed my puppy wasn't eating in the first few days, she eventually turned into a shaky, runny nosed, sick puppy with diorreah. She was sick and far too young to be away from her mum and the vet said she certainly had no type of spaniel in her and leant towards her being a Maltese, she certainly looked like one and grew no bigger than a small kitten. She also had some sort of genetic skin condition relating to inbreeding in non shed coat breeds. It cost me alot in vet fees. I tried to contact the "breeders" to let them know, they had taken their advert down and their number became a deadline. My puppy did not have an existing chip. I am pretty sure, looking back and being more older and wiser, she was more than likely stolen. It would explain alot of things, such as the disappearance after, the breed (despite being pleasantly taken aback she was a Maltese - my dream dog but never being able to afford one) not being as advertised. Although I am aware of a practice called "dogfishing" where sick puppies are sold. So please be careful.