I buy and sell all sorts of used tractors and implements. I started way back when I was still in high school as a way to suppliment my income. It's been very profitable over the years, as well as landing me some nice (CHEAP TOO) equipment for my own use. Recent years, it's getting increasingly tougher to find good equipment at good prices. The supply is drying up as the demand increases. The internet has also hurt in some ways. It has led to the general public being more aware of what a bargain is and where to find them.
Best places to find used implements;
Auctions....Estate sales seem (to me) to be the best source of better quality pieces, but consignment sales can net results too.
Craigslist.... Relatively new to the "game", CL is a good place to watch for bargains. Keep eyes and ears open. Not every item is described well. Sometimes you can sneak up on bargains simply because they aren't listed correctly. Also, post "wanted to buy" (WTB) adds. I've found a few things that way.
EBAY....You play the game, you take your chances. Enough said.
Drop hints to every salesman at the dealerships....Let 'em know you're in the market. Often they'll have leads on items that a potential customer might want to "move" but not at a price where they can take 'em in on trade.
Knock on doors.....Good way to find deals, good way to get chased off someones porch.....

If you see something that's been sitting un-used for ages, never hurts to ask. I found a killer deal on an IH 474 tractor that way last spring.
Move quick. Bargains don't sit around long. If something is cheap, buy it. You might not NEED it, but reselling can be profitable, AND, it brings potential TRADERS to your door. The more you deal with equipment, the more channels of opportunity you open.