DeeryLou said
Why pay a whole bunch of notes for a cat, and immediately ship him off to the “great wasteland”?
The amount of notes a cat sells for is a huge bit of context here. Cats selling for around 400-600 notes isn't anything much; it's extremely easy to get that amount of notes within a few hours of Adventuring or even passively over a few days of just doing your dailies.
For cats that are on the cheaper side, they're being trained up to be citied for the resources they give. A level 1 cat bought off of the CRB will give you 3 Denim's Traveler Packs if trained up to level 4. Opening these packs will give a total of 150 notes and 3 essence fragments. Citying them and opening the packaged gift will give another 150 notes and 1 essence fragment. In total, you can get 300 notes and 4 essence fragments from one cat with this method. If you sell the fragments for notes, using a ratio of like...1:400, then that's a total of 1900 notes from a cat currently.
So, these buyers are just doing their best to make a lot of money! Like Nightmare said, there is also an EF shortage right now, so people may just be after the EF this methods give. It's a slow-drip for sure, but with a ton of custom content to buy, the people are hungry.
If the cat is more expensive, it was probably buyer's remorse.