When I first became interested in playing the mandolin, I did hours of research on the internet looking for information; as a result, I found there is plenty out there. This guide covers the basic information I found.

The most important thing I can tell you is to ask questions. There are several very knowledgeable and helpful sellers of musical instruments on eBay. I bought my first mandolin (see picture)
a Starsun model A from such a dealer, saledepot. For example, he told me that there are only two or three factories in Korea that manufacture mandolins under numerous names. A Starsun is manufactured in the same factory with the same materials as Johnson, which sells for much more; as a result, he saved me a hundred dollars.

You will notice that there is a big difference in price between an A style and a F style mandolin.

The F is fancier, with a scroll-and-points body shape made famous by Bill Monroe, who wrote music history with a "new" type of music (named after his home state of Kentucky) called bluegrass. Bluegrass mandolin players favor the F because it gives a solid sound on rhythm (chop) chords, along with a clear tone needed for solos. 

A-style (tear drop shaped) instruments come in many varieties, from vintage Gibson A-styles to budget priced Asian copies. These A-styles are prefered by old-time and Celtic musicians who like a brighter sound that will cut through as well as blend with other instruments.

Both styles are available with f-shaped and oval shaped soundholes. There are many different opinions on which is better; basically, oval soundholes offer better tonal qualities, F soundholes have better projection.

The best advice: ask questions, choose an instrument that will sound good to you, and is one you can afford.