When shopping for a used 3-point garden tiller, your purchase should hinge on how much you are willing to spend, the size of your garden, and the condition of your garden. As with buying any used product, it is also important to evaluate the integrity of the machine you are paying for: make sure it is usable, optimally functioning, and will stay that way for as long you think you will need the tiller.
If you are shopping for a garden tiller, then you are probably not a novice gardener. Tilling one's garden is a wonderful way to improve the quality of soil via aeration and even incorporation of fertilizer and compost into your garden. Although this can be accomplished with the basic shovel, tillers accomplish this job quickly and efficiently. Further, tillers eliminate the painstaking chore of weed removal, as you can trust this handy gardening tool to chop and cut unwanted weeds in the process.
Keeping in mind the type of garden you are managing is critical for purchasing the kind of tiller that would best suit your needs. Small gardens, it is no surprise, require smaller tillers that can maneuver between narrower rows of plants; these tillers need not penetrate the soil too deeply. Larger gardens, however, require bigger, more powerful tillers that can do a more complete job breaking up soil and incorporating fertilizer, etc.
Also, depending on the quality of the soil you are working with, choosing between a front-tine or rear-tine tiller may be something worth considering. However, when browsing for used tillers, ask the assistant in the store to help you in your choice. They should be able to explain to you the differences between tillers, and can guide you in choosing the one that best suits your needs, as well as your budget.