I Am Not A Lawyer, ask a real lawyer, yada, yada, etc.
Basically what's stopping them is the GPL itself + Econ 101: they'd have to admit that it's just your code sold for an infinitely higher amount. Otherwise, you can (legally) force them to provide the source code to you (if you buy their app) and then incorporate it in your version.
This freedom is a foundation of the GPL and it is not considered stealing or otherwise bad/unethical within the GPL community.
Everybody gets to use what you wrote, and what someone wrote over it (modified). The source code is an integral part of a product in GPL and so is the right to redistribute it freely.
The only thing you would lose if someone forks your app is the ability to license to yourself their codebase under a different license (than GPL).
To explain this further, if you write something from A to Z, you have full copyright and you can do with it whatever you want, including licensing it to some people under the GPL, while you can license it to yourself under another license. However, when someone else has created a derivative work, you no longer have copyright over their edits of your software and you need their permission to use their edits under a non-GPL license (Your GPL rights are guaranteed by the GPL itself, since it is "viral" and it automatically spreads to the modified source, a.k.a. the derivative work).
Also, if you write the app yourself, you can release it under another license that forbids other parties to sell derivative works from your app. This way you would be legally protected from such problems.
In reality, things are actually quite different. In many cases, the owner of the store is either not cooperative, not a legal entity at all, operating in the 'shadowy' zone of law, in another country, where your copyright does not allow you to force them to take down the app, etc.
However, most big stores would actually go out of their way to oblige to the party that claims its rights are infringed. This is because copyright infringement laws are getting more and more (too much, IMO) stringent and the potential loss for a service provider that does not comply with a DMCA takedown far outweigh the losses of a legit app taken down by a mis-targeted takedown notice.
Of course, you should refrain from posting fake DMCA takedowns, since I believe this is a crime at least in the US, and even if it's not, you are less likely to be taken seriously, if your account spams takedown notices 24/7.