The link above takes you to a classic case of how it can all go wrong when you buy a pet online from one of these personal sales websites.
There are so many possibilities for problems which affect the buyer financially and emotionally and the animal in terms of health and welfare.
Many sales are fine but you really have to visit the seller at their home/facility when it comes to animals, I believe.
I know many purebred cats are sold online and shipped by air. A1 Savannahs did this successfully for many years and still do and it works most of the time. For me that does not make it any better.
The trouble with personal sales websites like Gumtree is that it attracts unscrupulous sellers and buyers. The “product”, a sentient being, is vulnerable and treated like a commercial asset with little regard for welfare.
Gumtree should be restricted to inanimate objects.