Getting a good used garden tiller is actually more complicated than deciding which new tiller (or tillers as the case may be,) would be the best buy. The only motivation that generally drives the search for a used garden tiller is price, especially if you're looking for a full size garden tiller. But, you have to consider more than just the initial price. Over time you could end up spending more for a used garden tiller than a new one.
The first and usually most difficult determination to make is how well maintained the current owner has kept his garden tiller. Looks can be deceiving. Maintenance and care make all the difference between a good deal or not on any used machine. Was it used only within the parameters that it was manufactured for or did the owner get "creative" with its use? Was it properly over-wintered or left out in the elements?
Certain things will stand out in this regard. If it generally looks beat up, you may not have to look much further to determine what kind of shape it's in. However, there are a couple of things to consider. While it may look well used, if the garage sale price is making it almost irresistible to pass up, examine the actual working parts to see what kind of shape they're in. Are the tines straight? Can they be sharpened? Are the bushings rotten or have they been regularly greased? For a gas powered tiller, check the spark plug; see if the engine starts right up. Check the handle fittings to make sure they're sound. A handle replacement can be expensive and may be more than you want to spend. If that all checks out and it's manufactured by a reputable company, then despite some exterior body rust and rock dents, it may indeed be a good buy.
The other consideration is your own mechanical skills. If you're a fix-it do-it-yourself type with the appropriate array of tools to work on a garden tiller, then buying a couple really cheap used garden tillers might be an option, so long as they're made by the same manufacturer and are compatible models. My neighbor is a garage sale junky. He's rebuilt more than one tiller by combining the best parts of several that he got really cheap. The end products were good reliable garden tillers.
There is one type of used garden tiller that is rarely a good buy used and that's a compact or mini-tiller, especially if it's electric. They're inexpensive enough to make it almost never worth buying used. There are exceptions, of course. I had an elderly acquaintance whose husband died unexpectedly in his sleep. He was one of those people who was very particular about his tools. He always purchased the best ones and kept them in excellent condition. When his widow got ready to sell the house and move in with her daughter, his tools went like hotcakes at the garage sale, and I can guarantee that every one of them was a good buy. So, these situations do come up, but they're few and far between.
In general, getting a good used garden tiller is a challenge, mostly because serious gardeners buy their tillers for the long haul. They maintain them and use them for years and years until the tiller starts getting too expensive to repair and maintain. By that time, it's only good for the recycler. Even then, often the owners are reluctant to get rid of them. So, it boils down to this: unless you're fortunate enough to have found an exceptionally good deal, or skilled enough to make any deal work for you, it's usually more prudent to invest in a good new one.
