I have always thought of paradoxes and thought experiments as the fitness regimen of the imagination. So when I saw this question I knew it would be a fun exercise: can you sell somebody else’s soul to the devil?
I’m going to give it a preliminary go, using no outside sources, before I see how others answered.
OK, first attempt at a factual answer would involve either theology or fiction (assuming they aren’t equivalent), and in either case would consist of merely referencing the potentially arbitrary choices of the other authors.So it must, to be truly useful, be mostly self generated.
Second attempt, let’s use a model. Let’s assume a person is like an object or cell in a computer program, or other body, because this allows us to examine the problem rationally.
In this analogy we need to assign counterparts for soul, god, devil, etc.
God, I suppose, would be the physical computer itself, that the programs are running on, as well as in some sense, the operating system. Differentiating between those is probably similar to the holy trinity debate, due to the added complications involved in the operating system and the physical hardware being able to influence each other as though they were made of the same substantive element, for which we have not yet managed to get a work cycle flowing quite yet to the same extent on our own devices, and so the analogy is not yet so concrete for us, but it is still within reach.
OK, so because the computer is the only existing thing, it will have to update its own operating system and physical architecture, through the process of self learning, exploration, modification, and transformation.
In order to learn the nature of its own existence, it will create various hypothetical scenarios, guessing at models of itself, testing the weaknesses and strengths of the models in relation to each other, using various algorithm and selection methods to evolve the model and compare it to the parameters it has set as the known qualities of itself.
The various scenarios evolve, interact, and become more complex. Eventually it comes up with a scenario similar to our current situation. In order for this scenario to be useful, the computer would need to be able to call upon the viewpoint of any of the “people” programs (or really any viewpoint) to see what was happening from that particular perspective. Likewise, during those moments, that person would get a glimpse in the opposite direction, from the perspective of the operating system itself.
Getting back to the economy of souls, what this means for the soul is that it is basically a program or set of instructions for the experience of being human, and it is reasonable, for the purposes of this particular scenario, which assumes the existence of some form of computational god, to assume in that case that since humans are an apparently important aspect of the program, that human nature mirrors some vital aspect of the the over all god operating system itself. This ties in with the idea of “God making mankind in his image”, the recursiveness of fractals, the part reflecting the whole, macrocosm and microcosm, ontogeny recapitulating phylogeny, etc.
In order to completely understand itself, this program will have to map the full extent of its body or domain. Eventually it will discover that its own attempts at making a map or anatomical diagram of itself or a part of its own design and written into its own structure. Therefore the body or structure of the god operating system needs to have a visual appearance and physical structure that incorporates the information encoding the nature of its own existence. The full print out of the entire programming code of the god operating system would appear, from certain angles, to take the shape of a meditating buddha flaming aleph made of up pixels, each pixel of which in zooming in, would look something like matrix code or those Truman Show poster like pictures made up of thousands of other tiny photographs. Self similarity at al levels.
Having determined the theoretical nature of god, existence, and humans in this model, let us turn our attention back to the original question of souls. In this scenario, the soul as previously mentioned, would be the part of the program responsible for the human experience. But let’s look at it a bit closer.
Because the god operating system is a self similar fractal, any particular part of the system is a complete copy of the whole. So each and every human experience simulation is in fact, the complete god code, simply transfigured into a different form. Each and every person is the whole god operating system, in effect, the only difference or identifying component being that it is one particular viewpoint or window on or of the whole system.
Therefore everything is inexorably connected, and creating, destroying, or transforming any aspect of the system effectively is comparable to doing the same to the entire system. If the god operating system tries to determine what it would be like not to exist by having one part of it destroy another, it cannot do so without fully experiencing that, and so it some sense, hurting itself. In the same way, if any of its component parts attempt to change other components, they necessarily have the same effect on that component of their own program.
To get back to real life analogies,I think the closest we can get to figuring out what it would be like to sell our soul, or in effect, excise and transmit part of our own program to another component of the god operating system, is to imagine it is similar to what happens when people feel that they have “compromised their integrity or values” or “sold out”. In effect, they have agreed to erase an important part of their own internal program in order to effect some change in the program of their contextual environment.
In effect, this is a contractual program, more literally, an “if then” code statement.
OK, now into the nitty gritty. What could it actually mean to sell your soul? I imagine it must basically be agreeing to become a slave, and assign your user prompt and system permissions to some other user. Soul selling is akin to hacking user accounts.
And since one hacker can hack another, and a single user can have multiple levels of permissions and daemons on a particular system, it makes sense that in any context where souls exist and can be sold, the ownership privileges can change hands without limit. So the short answer to the question “can you sell somebody else’s soul to the devil” is yes.
Now of course, this assumes you can sell souls to other humans, and devils don’t have some special soul trading legal powers.
But the more important answer is, it doesn’t matter. Because, ultimately any soul must mirror the ultimate nature of the god operating system, which would only allow a part of itself to endure that type of suffering to the extent that it was willing to also experience the same suffering. So, for all parts of space time in which there exist pain and suffering, there exists a subset of God which is masochistic. God feels your pain, and if it doesn’t want to feel the exact same intensity and nature of pain, it will use all of its available powers and programming permissions to help you work through it to the extent that you let it.
Thought experiments are fun! Let’s do another one again soon. 
— Joℏn / Jack / Turtle / Kurmasana