Just a guess what does the variable json contain after

var json = Sys.Serialization.JavaScriptSerializer.serialize(obj);?

If it is a valid json object like {'foo':'foovalue', 'bar':'barvalue'} then jQuery might not send it as json data but instead serialize it to foor=foovalue&bar=barvalue thus you get the error "Invalid JSON primitive: foo"

Try instead setting the data as string

$.ajax({
    ...
    data: "{'foo':'foovalue', 'bar':'barvalue'}", //note the additional quotation marks
    ...
})

This way jQuery should leave the data alone and send the string as is to the server which should allow ASP.NET to parse the json server side.