Get whatever camera you can afford, but the important thing is to actually hold the camera. It should feel comfortable in terms of size and weight. Controls should also feel comfortable. If you have small hands and get a large, heavy camera it will always feel like a brick in your hands and you won't enjoy using it. The reverse is also true. A small camera in big hands will mean fumbling around and frequently hitting two buttons at once.
I'm a Canon person, but there's no appreciable difference between Canon and Nikon these days. It's whatever you're most comfortable with.
Remember that when you buy a DSLR, what you're really investing in is the lenses. Cameras, in the final analysis, are just recording devices. Granted the higher end bodies offer more recording tools, but at the entry level, virtually anything in the $800 to $1,200 range will get the job done. The lenses are what matters. Buy the best lenses you can afford. Lenses retain value and are the thing you keep as you move from body to body. Canon and Nikon both have extensive lens lines, both pro and consumer.
Consumer-level lenses are much less expensive, not built for heavy use, and deliver good quality images if you stick with the Canon or Nikon brands. They have limitations, but usually work for the casual shooter. Professional-level lenses (Canon's L line, for example), are built for heavy use, have much higher quality glass and construction, and deliver top-notch images, assuming the organism pushing the shutter button does his/her job. If you can afford them, professional-level lenses are well worth the money.
Third-party lenses, such as Sigma and Tamron, can be good values. Sigma's EX line is their "professional-level" line and, for the most part, very good. Sigma's consumer lenses are very inexpensive and the images they produce will demonstrate that you get what you pay for. I use their 50 and 180 EX macros and their 24-70 EX. The rest of my lenses are Canon L lenses.
If you are going to buy a macro lens, start with the 100-mm focal length. It'll give you enough reach and enough aperture range to get most macro shots. Canon's 100-mm macro is, of course, excellent. Sigma makes a 105 EX macro and Tamron makes a 90 macro that are both excellent. With the Sigma lens, it has external focusing, meaning that the "snoot" extends as you focus. I owned one of those lenses and never thought that it was a factor. For some, it's a big deal.
I would never consider buying camera equipment from E-Bay. Too risky. I've only purchased one used lens, but it was from a friend who treats his equipment just like I do, so I never worried about what I was getting. If you buy used, be sure you actually hold and use the piece of equipment so you'll know what you're buying.
Gary