The most important thing is getting a bike that fits you. No bike will give a satisfactory ride if it's the wrong size.

Next, if you know nothing about bikes, you should try to find one that is in good shape and doesn't need new tires, cables, brake pads, chain, gears, etc. Those little bits can add up to a lot if you take it to a shop to get all the work done. It's hard to tell the condition of those bits on ebay but if you ask the seller they might be able to help.

If you really know 'nothing' about bikes, I would try to find something local that you can ride before venturing out into ebay. Ebay prices usually favor the seller and unless you know what you're getting you could be easily disappointed.Try your local bike shop and try different sizes and styles of bikes even if you're not going to buy one from them. If you do get a bike elsewhere, you can always patronize them for the service or parts you will eventually need. Good luck!