Before you begin your search for a used milling machine, you will need to decide what type of milling machine you need. Plenty of options exist for different applications, and the size, intended purpose, and orientation of the milling bit will have an effect on the usefulness of the tool. Do a fair amount of research about the different types of machines and find the one that will work best for your purposes. Once you have determined your needs, consider your budget for purchasing a used milling machine, as the costs of these machines can vary significantly.
Determining the overall condition of the used milling machine will be difficult, and it is important for you to remember that even after a visual inspection and test run with the machine, you may end up buying a machine that does not work well or one that breaks shortly after purchase. If possible, ask the seller about any warranties that may still apply to the machine, as well as if the seller will offer a return policy should the machine stop working immediately after purchase. Such assurances are usually easier to obtain if you are buying from a retail store; it may be impossible to get a warranty or return policy for the used milling machine if you buy from a private seller.
When you inspect the machine, look for visible signs of neglect, damage, or other indications that the machine was not well cared-for. When buying from a private seller, be sure to take note of the condition of the seller's work area; look for well cared-for tools and a clean workspace. The used milling machine is less likely to incur damage in a space that is well-kept, though this is hardly an assurance that the mill is in good working order. Whenever possible, test out the used milling machine with scrap materials to ensure it works properly. Take note of the ease of function, the sound of the motor, and the condition of any bits, moving parts, and so on.
Try to do as much research as possible about the milling machine models you are considering purchasing. Find out any information about recalls or common repairs that need to be done to that particular model, and be sure to ask the seller about these problems or recalls. The more you know about the milling machine, the more likely you will be to ask educated questions and to get honest, clear answers.