if someone goes ahead and pays this one-time fee to purchase a MIT license to gain access to code which is otherwise available through GPlv3, what's to stop them from uploading the MIT version to Github with the MIT license attached. Wouldn't that be within their rights and perfectly legal?
Yes. The MIT license is clear that you can do whatever you like so long as you keep the MIT license notice intact.
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
If they add additional restrictions then it is not the MIT license.
...and wouldn't that be circumventing the intent of the party selling the MIT license?
I'll guess that the intent of the author is to have their copyleft cake and eat their commercial software, too, to mangle a metaphor. Many projects are skittish about using GPL code in their project for reasons real and imagined. Offering their code under a more compatible license is a way to handle that and make a little money.
They could have chosen a different license for commercial use, for example MySQL has several commercial licenses and exceptions, but they didn't. Whatever their intent was they chose to sell an MIT licensed version and that has consequences.
Note, the MIT license only applies to that specific version of the code, a snapshot which will presumably be unsupported. Later GPL releases would not be covered. They could choose to make a fork, but they would have to be careful to wall themselves off from the GPL code.
Given that they would be releasing an MIT licensed snapshot of what is already available under the GPL there's little harm done. And if someone wants to run with it and make an MIT licensed fork, those changes can be incorporated into the GPL project. This is in the spirit of the GPL and Open Source.