Re-skinning
The biggest problem that I can see here then is what is known as re-skinning.
This is where the seller would find a buyer for their app, but then change the look of it by changing image assets, colours, sprites etc and give it a new name.
For example then, the theme of the app might be something like aliens with planets and space etc. and then the developer decides to make a cowboy theme and swap out the aliens for horses and swap out the planets for deserts.
All the seller needs to do now is to change the name of the app and it would seem to be unique at first glance until you realise that the are ten’s of apps out there in the app stores that is the exact same code base to the one that you have just bought.
The trouble here is that the seller makes a sale, but then still has a copy of the original code.
I believe that when someone sells code, they are then selling the rights to use that code.
But of course Flippa seems to be a place where no one cares about their moral standards.
Flippa does try it’s best to regulate the site and I have purchased apps from a seller where he was then removed from the site.
The problem here was if I ever needed to communicate with the seller again then I was out of luck.