Basically, I was just wondering if I release a piece of open source software under the MIT license, does that mean that someone could take it and sell it onto people without doing any modification to it?
Yes. The license itself is really just one sentence long, and states explicitly that this is allowed.
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.
This is of course not legal advice, we're talking hypothetically...
Yes, this is possible, and for the most part it's fine.
The idea behind many Open Source licenses is that this effect is nothing to worry about. You as the author intend not to make money from selling the software yourself, so you open-sourced it. You presumably are also making it very easy for the public to access your sources.
Hence, if someone tries to sell your software, they will have no customers unless whatever additional service they add is worth it. For example, in the good old days before ubiquitous free internet access, it would have made sense for someone to provide CDs of large software distributions (Linux distros, (La)TeX collections come to mind); and they would certainly ask for money even though they "only" burned it to the CD. You were then paying the CD, the overhead of production etc., not the software per se. Not a cent of those payments would go to the original authors. If the original authors wanted to earn money this way, they were free to sell their own CDs (maybe with added benefits, like being faster with new releases, adding expert support on top, whatever). So the re-seller did not hurt the authors in any way.
Another example: you can develop a nice MIT-licensed command-line "engine". Everybody is free to grab that, add a fancy GUI on top, remove the command line interface, and publish it binary-only/closed-source, provided they include a note about your engine alongside it. (Compare https://softwareengineering.stackexchange.com/a/189704/219913 for a discussion how this relates to other licenses like BSD or GPL). They can make it so nobody would even notice that your stuff is in there if they'd not read the lincense.txt or "About" button.
Even if you think that this were rude or unfair, it remains that a potential buyer would in fact pay for the GUI, the bundling, etc.; and a potential buyer who does not need it still has access to your open-source bits. So if a buyer thinks the added functionality is not worth it, they can easily circumvent it - the re-seller cannot take anything away from you.