First of all, be careful if you are buying second hand. Ellipticals do take a lot of stress when they are in use. They can therefore be prone to breaking down. If you are buying from a healthclub, gym, or person that has been using the equipment everyday, then the chances are that it may need maintenance in the near future.
Always check the warranty on the elliptical machine you are buying. The best machines will have a long warranty such as 2 years parts and labor. On cheaper machines there is often a very short warranty. Insist on getting at least 12 months warranty, any less and the quality of the machine is questionable.
Stride length is a factor that will affect how comfortable the machine is to use. On cheaper machines the stride length is often 16 inches or even less. The better quality machines will have a longer stride length. This is particularly important if you are tall. You will find that the shorter lengths will feel like a 'choppy' ride. You should ensure that you get a machine with a stride length of at least 18 inches. If the whole family is likely to use the equipment you can get a machine that has an adjustable stride length.
Be prepared to pay a little extra to get an elliptical machine that is well built and configured. This will reward you with many years of enjoyable exercise.
Discover the best elliptical machines at http://www.my-elliptical-trainer.com. The great elliptical vs treadmill debate: Which is best?