Is there any possibility of adding licensing of some sort to the files? What is in place to keep someone from downloading everything here and selling it as a library, or similar?

Oh, and there IS one thing that prevents anyone from downloading, stealing, or selling off your files.......YOU NOT UPLOADING THEM. I believe it is your choice knowing how this site is set up to work AND you can even take them down. I personally go about it the same way a lot of others do on here. I don't upload anything I don't want public, or if I want to easily share it with another member i set it to private and send a link.

I'm thinking of uploading some of the work I would not be too happy to be misused. I think that there will be no assembly files, only assemblies saved as a single solid file, acceptable and usable for rendering, but without functionality it has as an assembly. That is some sort protection. The other thing is that inventing and developing new things is what I waste most of my time with, and by the time someone figures out what my intention and point was with the design I made public, I already have an improved and even better solution and design. What I have to keep away from public (stuff regulated with disclosure agreements), stays away from public. Simple as that. The "secret" is that just as long as no one else knows about it... :)

All of the stuff I've made is in educational edition anyways so no one can really do anything with it, as others have said don't upload it if you don't want others to take it and pass it off as there own

copywrights and/or provisional patents are not expensive to get...i have several designs that i am filing provisional patents on before i will post them up here

unfortunately you cannot control anything that goes to public. if you don't want to share it with the public then don't upload it or if you want to share it with few guys here you can do that and keep it between you as a shared file.
also there are several ways to present your model without uploading the original file such as 3dpdf, Screenshots, video, etc.

I believe the grabcad provide the best for your files, and I agree with Aaron, it all depends on you to share in positive terms.
grabcad the rest of the answer to the rule download. in order to avoid unwanted.

I have a slightly similar question:
If I download models from GrabCAD and use them to make renders which I sell, will I be liable? Will the original author be able to sue me for a portion of my earnings?

Usage rights are clearly stated in the Website Terms of service.

"6. USER SUBMISSIONS
If and when you submit, post, upload or otherwise make available Content (such as CAD files) on or through the Site for public viewing ("User Submissions") you hereby grant:
6.1. Stratasys and all Stratasys Affiliates a worldwide, non-exclusive, royalty-free, fully-paid, perpetual, irrevocable, sublicensable (through multiple tiers of sublicensees), assignable, and transferable license to use, reproduce, copy, modify, adapt, arrange, translate, create derivative works of, and publicly display, perform and communicate such User Submissions, for the sole purpose of making available, improving, and/or promoting the Site and/or Site Materials (the "User Submission License"); and
6.2. each user of the Site a worldwide, non-exclusive, royalty-free, fully-paid, perpetual, irrevocable, non-sublicensable, non-assignable, and non-transferable license to use, reproduce, copy, modify, adapt, arrange, translate, and create derivative works of such User Submissions for such user's own benefit and non-commercial, internal use (the "Cross License").
You hereby represent and warrant that: (a) your User Submissions will not infringe, misappropriate or violate any third party's Intellectual Property Rights, moral rights, privacy or other personal right, or any Law; and (b) you have, and will maintain during and after any termination of this Agreement, all licenses, consents, permissions and approvals required to grant the User Submission License and the Cross License. Furthermore, you hereby irrevocably and unconditionally waive (and will caused to be waived), and covenant never (whether during or after the Term) to assert against Stratasys, Stratasys Affiliates, and/or any user of the Site, any moral, privacy or other personal right in or to your User Submissions. The User Submission License and the Cross License shall survive any termination of this Agreement. User Submissions made publicly available will not be deemed (or treated as) confidential or sensitive to you."