First, be wary of buying a unicycle from e-bay, there have been some posts on unicyclist.com forum about bad deals.
You need to do some research; where are you? unicyclist.com has branches in several countries and sells unicycles that are well designed, so check that out.
What are you going to do with it? i.e. just learn to ride a bit or maybe get into muni ('off roading') or do distances on it?
Many people will say 20" wheel for beginners- it's an out of date cliche; a 24" has many advantages especially if you want to ride in the streets or off road.
I wrote a FAQ covering some of the issues with different wheels sizes, its's here: -
http://www.poiinthepark.net/phpBB2/viewtopic.php?t=19There's links to some excellent unicycle resourses and links to some very inspiring Kris Holm vid clips.
If you're buying new you can get a cheap uni such as: -
http://www.unicycle.uk.com/shop/shopdisplayproduct.asp?catalogid=511Fine to learn on, but note from the description that it has 'lollipop bearing holders'; this means that eventually it will break- the frame will develop cracks where the bearing holders fit in.
If you can afford something like this: -
http://www.unicycle.uk.com/shop/shopdisplayproduct.asp?catalogid=485then consider it. Nimbus ('Yuni' in the U.S.) unicycles are excellent for the price, my 24"x3 Nimbus has ridden a lot of miles, both on and off road, over the past few years, it's still in excellent shape.
You'll see from the description that it's 'entry level'. that means that you can't land 4 foot drops on it without it breaking. However, for street riding and off roading without such extreme drops it's a superb machine.
It has 'main cap' bearing holders, which means there are no frame cracking issues. (sorry I've no pics of the difference between 'lollipop' and 'main cap' bearing holders, just be aware that there are issues with lollipops)
Read the FAQ, check some sites, take notes; feel free to post further questions and I'll try to answer them. (Please don't pm me; I've been getting a fair few pm's asking about unicycle stuff and I'd rather answer on a public thread cos then more people can get the info).
Good luck, unicycling is great, especially if you get into the outdoor stuff, it can be as life changing as spinning