First of all welcome to Tnet. Second, I don't think there is any item in particular to look for. The "finds" at yard sales and such are very diversified. I have probably 2-300 books on different types of collectibles and antiques and I am far from an expert, nor do I have everything memorized. I'd need a photographic memory for that. I do have a good eye though for certain things. Mostly I buy what I like. If your gonna do it for profit then my advice would be to read, read, read and then read again. You can find alot of information on the internet. Google is a wonderful thing if you use the right search word/s. Visit your local antique malls and stores and look around, ask questions and familiarize yourself with things. Also there are many items that aren't even old that can be found for bargain basement prices. Here is one guys advice on buying for profit-
http://www.ehow.com/how_2161008_buy-acquire-sell-profit.html.