How can I use jQuery to style /parts/ of all instances of a specific word?

To do it reliably you’d have to iterate over each element in the document looking for text nodes, and searching for text in those. (This is what the plugin noted in the question does.)

Here’s a plain JavaScript/DOM one that allows a RegExp pattern to match. jQuery doesn’t really give you much help here since selectors can only select elements, and the ‘:contains’ selector is recursive so not too useful to us.

// Find text in descendents of an element, in reverse document order
// pattern must be a regexp with global flag
//
function findText(element, pattern, callback) {
    for (var childi= element.childNodes.length; childi-->0;) {
        var child= element.childNodes[childi];
        if (child.nodeType==1) {
            findText(child, pattern, callback);
        } else if (child.nodeType==3) {
            var matches= [];
            var match;
            while (match= pattern.exec(child.data))
                matches.push(match);
            for (var i= matches.length; i-->0;)
                callback.call(window, child, matches[i]);
        }
    }
}

findText(document.body, /\bBuyNow\b/g, function(node, match) {
    var span= document.createElement('span');
    span.className="highlight";
    node.splitText(match.index+6);
    span.appendChild(node.splitText(match.index+3));
    node.parentNode.insertBefore(span, node.nextSibling);
});