The BtnSellPart does not have an OnClientClick handler and there is no JS function in Locate.aspx that opens a RadWindow.
Also, see line 89 of Checkout.aspx in the sample I am attaching, it had an unclosed <tr> tag which causes invalid markup. I have also added the proper way to get the RadWindow reference that is shown in the first article I linked (lines 25, 58, 65). I also moved the script manager to the beginning of the form where it belongs in WebForms.
I advise that you compare my sample with your project and migrate the working code so you can start using it.
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