Don't be shy about inspecting an item real closely - being stealthy or appearing disinterested means nothing. Sometimes you can't always see those broken welds and cracks just by giving it a walk-by, and it saves you some pain later on.
Just like everyone else, I shop for new prices and discount according to age and condition. I have also gone to auctions with no intent of buying anything, but using it as a test to see what the going prices are for implements in my area.
Best advice is to buy it when you don't really need it. Then you never get your shorts all knotted up about winning a bid because you "just gotta have it now". Takes away the buy-it-now pressures, and you never usually overbid on something you don't plan to use for another 6 months. If the bidding is making this a pretty good deal and you know you'll need (want) that item for Spring planting / Fall harvesting, etc....then have at it and low ball your bids because you have plenty of time and other auctions to attend in the mean time - right?