First off, bear in mind I don't mean to sound cruel or mean...
But a "simple" LDB for that would require a tun of work, you're far better off by tapping into the Chat itself and parse those, I think there's even an event fired when a buyer is found. If you look through the events related to the AH you will find it. Give it a try to write it yourself, you won't believe how great it feels when you accomplish it
(Ps. I doubt someone will just write it up for you, you could look at addons that look for specific chat messages and adapt from there.)