jQuery: check if text exists in another element

You can use :contains() instead of :has():

$("#results a").live("click", function(event){
    var seed = $(this).text();
    if ($("#seeds:contains(" + seed + ")").length){
        alert("already have it");
    }
    else {
        // other code
    }
});

This checks if the #seeds element contains the text that the link clicked contains.

If you want to find any elements that are descendants of the #seeds element:

$("#results a").live("click", function(event){
    var seed = $(this).text();
    if ($("#seeds").find(":contains(" + seed + ")").length){
        alert("already have it");
    }
    else {
        // other code
    }
});

Here is a demo: http://jsfiddle.net/5wE3p/

Notice that I am checking the length of the returned set of elements, if there are no elements then zero will be returned and the if statement will resolve to false, if there are any elements then it will resolve truthily.

Also note that :contains() is case-sensitive: http://api.jquery.com/contains-selector/

Side-Note

If you are using jQuery 1.4.2 or greater then you can use .delegate() rather than .live(). The main benefit to .delegate() is that you can select your root element. So if for instance, #results, is always present in the DOM then you can use it as the root element rather than the document element (which is what .live() does):

//it's this easy to change to `.delegate()`
$("#results").delegate("a", "click", function(event){
    var seed = $(this).text();
    if ($("#seeds").find(":contains(" + seed + ")").length){
        alert("already have it");
    }
    else {
        // other code
    }
});

If #results is added to the DOM dynamically then you can find it's parent and use it as the root element (... as long as it persists in the DOM).

Update

If you want to only match the exact text of the link then you can iterate through the descendant elements of the #results element and check the text of each element:

$("#results a").live("click", function(event){
    var seed  = $(this).text(),
        found = false;
    $.each($('#seeds').find('*'), function () {
        if ($(this).text() == seed) {
            found = true;
        }
    });
    if (found){
        alert("already have it");
    }
    else {
        alert('that\'s new');
    }
    return false;
});​

Here is a demo: http://jsfiddle.net/5wE3p/1/

You can also use RegExp to match the value:

$("#results a").live("click", function(event){
    var regexp = new RegExp("^(" + $(this).text() + ")$", "i"),
        found  = false;
    $.each($('#seeds').find('*'), function () {
        if ($(this).text().search(regexp) > -1) {
            found = true;
        }
    });
    if (found){
        alert("already have it");
    }
    else {
        alert('that\'s new');
    }
    return false;
});​

Here is a demo: http://jsfiddle.net/5wE3p/2/

The important part here is the creation of the RegExp: var regexp = new RegExp("^(" + $(this).text() + ")$", "i"). Notice the i flag which sets the RegExp to search case-insensitive. Also the ^ matches the beginning of the string and $ matches the end, so this look to see if the current text being checked is the exact same string as the search term (except that it's case-insensitive).