Passing parameters in a jQuery ajax call to an ASP.NET webmethod

I know there are more threads about this but they dont help me and I'm going insane here!

I wanna pass some parameters in to a web method using jQuery Ajax.

var paramList = '';
for(i = 0; i < IDList.length; i++){
    if (paramList.length > 0) paramList += ',';  
        paramList += '"' + 'id' + '":"' + IDList[i].value + '"';  
    }
    paramList = '{' + paramList + '}';  
    var jsonParams = JSON.stringify(paramList);


    $.ajax({
        type: "POST",          
        url: "editactivity.aspx/UpdateSequenceNumber",          
        data: jsonParams,
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function(response) {

        }
    });

In the ajax call, if I put data to paramList I get the error: "Invalid web service call, missing value for parameter: \u0027a\u0027."

If I put data to jsonParams I get the error:

"Cannot convert object of type \u0027System.String\u0027 to type \u0027System.Collections.Generic.IDictionary`2[System.String,System.Object]\u0027"

If I write out paramList, it's in a correct JSON format like {"id":"140", "id":"138"}

If I write out jsonParams, it's in an incorrect format like "{\"id\":\"140\",\"id\":\"138\"}"

The web method: (it doesn't do that much yet..)

[System.Web.Services.WebMethod]
    public static string UpdateSequenceNumber(string a, string b)
    {
         return a+b;
    }

What am I doing wrong? Can't seem to get this JSON thing right.

UPDATE:

After some help from the first answer I now send {"id":["138","140"]} in the AJAX request.

The web method now takes a string called id as the parameter instead.

[System.Web.Services.WebMethod]
public static string UpdateSequenceNumber(string id)
{
     return id;
}

Now I get the new error:

"Type \u0027System.Array\u0027 is not supported for deserialization of an array."