How to choose an alto trombone and what to use it for:
Many eBay sellers claim they're selling an alto trombone. Most of these are Bb tenor trombones being sold by unknowledgeable sellers. It's a little difficult, especially for a novice, to tell an alto by looking at it, but they are a short trombone (between the tenor and soprano) in Eb or F.
A good choice for a first alto trombone is the Amati which is a good knock-off of the Bach. For the money and quality of the horn, the Amati is a better deal.
The cheap Selmans being sold on eBay are a good deal for the money if you are buying an alto to check them out or wish to put together a trombone choir. Despite their cheap cost, they are a good student-model horn.
The alto trombone has not one but two uses: in the 19th century classical repertoir it is used to play 1st tenor trombone parts, but in earlier and modern repertoire it is often called upon the play alto parts in the same range as the Horn and Eb tenor (alto) horn.
This means using two different types of mouthpieces- a small tenor trombone mouthpiece like a Bach 15EW and a bass trumpet mouthpiece like the Schilke 40B.
This in turn means two things- a beginner or intermediate player will be unable to play at the level demanded by the alto trombone on the high repertoire, and for playing 1st trombone parts, obviously the player will have to have good range and chops.
However, the for the student who likes a challenge, learning to read trombone bass clef is a snap. To start with, you can begin by "cheating". An old trick is to read the piece as though it were in treble clef and add three sharps. This is the same trick used by many Eb tuba players.
After that, just remember to bring both an alto and tenor, saving the tenor for music that dips below the alto's range. Have both horns mounted on stands, ready to go.
As far as positions go, the positions on all single trombones are the same. It's just that the size of the positions gets smaller as the trombone gets smaller. To say that the alto's positions are "out" is nonsense. If you play G bass, tenor, alto and soprano trombone on a regular basis, over time you'll be able to switch without even noticing. It's just a case of doing it.
The alto trombone is not "rare". In fact, it has made a huge comback, and all but a few manufacturers are again making altos. Tenor trombone players often gripe about them, but so do Bb and Eb tuba players gripe about each other's instruments. The bottom line is, the alto trombone is just another brass instrument, and if it's right for you, play it.