Things you need to know before you buy a mbira are

SIZE

Mbiras come varying physical sizes, metal strength, tonal qualities and age. The physical size is dictated by the gwariwa or soundboard. Smaller soundboards  are generally well suited for people with small to medium hands while larger sound-boards are suited for people with bigger hands. Large sound boards tend to be or larger. For first time purchasers, it is important that you get a mbira that fairly matches the size of your hands

TUNING

This is often difficult to ascertain when one purchases the instrument by mail. Experienced mbira sellers check the tuning and the key stability before disaptching. They they also make sure the mbira is  securely-packed for shipment. This is particularly important for new students who are not familiar with the scale of mbira. On the average, a package send through the postal service or any other carrier, is dropped from a height of 6 feet at least 4 times!

You also need to know the key arrangements

Basically your keys must have a lonian scale DO, RE, MI, FA, SO 

Age

Old used mbira have soft keys and sound better than new mbira and have artique ethnic value. With continuous playing, the New instrument  will get better. The more playing time the mbira gets, the more the metal keys 'soften' and its tones get louder, longer and more pronounced. A played or used mbira has smoother key-ends has a better feel and sounds more established. It also has the advantage that the keys and tuning has stabilized.

Mbira Maker

Mbira making is an art and some artist are better than others. It is impotant to know the person who made the mbira and an experienced mbira seller always buy from best artist.