If you do not already have a camera system, I would recommend Nikon. I've been a Nikon user for years and I've used Canon as well. Both are excellent systems but my main reason for prefering Nikon over Canon is how easy they are to use. Settings that I change constantly from picture to picture are right there at my finger tips. I'm sure once you're used to a Canon it's just as easy, but it just seems that Nikon are more no-nonsense than Canon. Just my opinion, and an opinion that was formed after using both.
I have a photography business and I would not recommend buying anything you're planning to try and make a profit on from eBay. Any "retailers" you find online that have a deal that is just "too good to be true", is probably because it is. I bought my first DSLR on eBay and fortunately didn't get burned. But almost everyone else I know that tried to buy there has.
I always buy new from Adorama.com. They are a really reputable vendor who if you get a product from them and within their return policy decide that you don't like something, you can send it back, no questions asked.
As far as Nikon goes, I would recommend getting a D200 (10mp), or the new D300 (12mp i think). The D80 and the D40(x) are nice cameras, but they're not professional grade. They're not built to professional durability standards, and they don't use CF memory cards which are the professional standard. I personally have 2x D200's and LOVE them. I have them both with the optional vertical grip (which allows use of 2 batteries at a time in them instead of 1 without the attachment). This attachment makes taking vertical full body shots so easy!!
If you're going to be shooting mostly people... get a Fuji. They use Nikon lenses and have the absolute best color for people pictures of any DSLR available. Only bad thing, their operation is soooooo stinking SLOW!! The menus are confusing and every function on the camera goes at a snails pace. But the picture quality is amazing.
If you're on a budget, I would recommend buying really nice lenses (because lenses will stay current a lot longer than your digital body will) and buying a Nikon D70s. It's only 6mp, but megapixels don't make better images, they make bigger images. The more megapixels there are in your image, the bigger print you'll be able to make. (I shoot sports all the time with a Nikon D2h with it set on 2mp, and I print 5x7's and 8x10's all day with no distortion or pixelation to the images.) But yes, spend your money on the lenses and look into buying a used or refurbished D70s from Adorama (www.adorama.com). The D70s uses CF memory cards so whenever you have made some money and are ready to step up to a professional grade camera (or pro-sumer as the D200/300 would be) you won't have to reinvest in a whole new format of memory cards.
Not exactly sure what you're planning on shooting, but I shoot a lot of weddings, portraits, and sports... here are the Nikon lenses I use for all of my jobs:
Nikon 10.5mm fisheye
Nikon 12-24mm f/4
Nikon 17-55mm DX f/2.8
Nikon 70-200mm DX VR f/2.8
Nikon 80-400mm VR (use this for sports)
Which ever system you decide to go with, plan to stick with it for a long time. You'll be investing a lot of money into lenses and bodies and it'll be money wasted if a year from now you switch over to the other and then have to rebuy all your lenses for the other system.
Another thing... whatever you do... do not let some camera store jockey sell you on 3rd party lenses. Camera stores make more money selling you Sigma, Tamron, Tokina, etc. lenses than they do selling Canon or Nikon. Whatever system you have, buy those lenses. Canon and Nikon know how to make lenses for their cameras, and they know how to do it better than anyone else. Also, their glass is higher quality. You'll get what you pay for in buying the brand that goes with your camera.
Hope I was of some sort of help.... Just speaking from my own experience in getting situated with my own business and gear.
Don't forget when you buy, you'll need memory cards, I recommend SanDisk and Lexar, as I'm sure any other pro would do the same. Don't mess with off brands. Those are more prone to data loss than anything else. Also, you'll probably need a flash.... most cameras have them built in, but they generally don't sit high enough and can cast a shadow in the bottom of your pictures where the flash isn't reaching over the end of your lens. Also... buy spare batteries. When I bought my D200's I bought 3 batteries for each to make a total of 4 for each since I was planning on using the vertical grip attachment that allowed for the use of 2 batteries at a time.
Good luck!!