Telerik MVC extensions are open-source under the GPL, so yes, you can use them and sell your product if you follow the license restrictions. The codeplex website says the latest version is Q32011, so they are not the most modern.
Telerik MVC Extensions on Codeplex
If you need a more current version (which is now not approved by Kendo, and I believe that support ends with Q32013) or support, you need to purchase a license.
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