First of all you should know the use of the baler that you are looking for. If you need a hay baler then look for a hay baler in the context of its quality, usage and price. Since hay balers are not very often used, their use is only during harvesting crops, the quality of the material can be degraded. A baler for example has too much rust on it, appearing as if it has not been used for years. So think of the quality while buying a used baler.
Another factor that can be the most important depending on the quality of the baler is the cost factor. Since you are looking for a used baler to save some bucks, you should give the factor importance while making decision for purchase. Given the baler is not very often used in agriculture setting, its price can be fast lowered with the passage of time.
You will also need to consider the size of the baler. The balers in agriculture and industry use come in different size and baling material accordingly. So you can make your mind on what size of baler you need and then look for a used baler that comes up to your requirement. If the size does not matter for you, you can pick any size and thus leave the matter of buying on other factors.
Think of accessories with it, and their quality. For example, all balers have kickers installed inside them that push the bales out once the material is baled up to a size. There are some other accessories attached to them, and you should make sure that you know of them all and have them intact at the time of buying.
You can read about the balers of different brands online, in reviews sections or otherwise. Considering reputation of the manufacturer is important as it can give you some peace of mind on the quality of the machine or otherwise you can decide against buying the baler. If you already know about the balers, it will be easier for you to make a decision and if not, it is better you first gather some information and then visit the market to make a purchase.