Well, there are literally dozens of websites that address the general issues you might face and the advantages of going one way or the other, as well as many, many threads here that cover the topic--I suggest poking around and seeing what you find.
Here's a few things I'll mention in regards to your particular situation, though:
1) Depending on your simple photo editor needs, iPhoto (which comes with every Mac) might be ideal for you. If all you're doing is cropping, minor color correction, and exporting, it's nearly perfect. Since you probably deal in volume if you're a fulltime eBay seller, Graphic Converter might also be a good choice for you--it's got more power (a poor man's Photoshop, really), it's not expensive, and it has some good batch-processing features that might benefit you.
2) I don't know what the state of eBay listing tools on the Mac is. Probably not as good as on Windows, but it might be sufficient. Perhaps someone else will chime in.
3) There is a version of QuickBooks 2006 Pro for the Mac. I have not used it, and it's not quite feature-identical to the Windows version, but it is out there. I believe every Mac comes with QuickBooks 2006 New User Edition, a simplified "trial" version of sorts.
3) Your basic web apps are all available and will work fine. Apple's Mail for e-mail is a very good program and a lot of people like it, but there is also the free Thunderbird which is the same as the Windows version, Microsoft Entourage for a big fancy management-oriented app, the old standby Eudora, as well as a couple of others. Browsers include Firefox, which runs well, Camino a "Mac-ized" version of Firefox that is prettier and generally faster (it's what I use), as well as Apple's Safari, which is a decen browser as well. Oh, and Omniweb, of course. There is an old version of IE available, but it's junk. Then again, so is IE6 on Windows (I say this as a web developer and IT support guy--none of the businesses I work with use IE on Windows).
4) Hardware longevity is hard to call, but I can tell you that I know several people who have used Mac laptops as their primary computers for years without any difficulty. I can't say the same of the one family I know of who uses Dell laptops as their primary computers--they had major hardware problems after a while. Actually, "used" would be better--their second-hand experience with Apple laptops and firsthand experience with Dell's lead them to switch a few weeks ago. Buying the extended AppleCare 3-year warranty will give you peace of mind, but Apple hardware is generally built well--at least on par with higher-end Windows hardware.
5) With any new iMac or MacBook/MacBook Pro you have the option, of course, of running Windows if you don't like the MacOS or need to use Windows for one reason or another. They can boot directly into XP (you hold down a key at startup to select the OS), or via a $40 program called Parallels you can run a Windows "virtual machine" from within the MacOS--basically a window with Windows running it in. This is very convienent (I use it for web development--testing on IE), since you don't need to restart, and in fact if Windows has a problem you can restart it without affecting anything else you have running on your Mac.
You will need a Windows license to do this, but you may already have one. Otherwise, it'll cost you a bit, but it's a one-time expense.
6) I recently did some price comparisons between Apple and comperable Windows laptop hardware for friends. What I turned up was that while you can get, say, a functional Dell laptop for less than a similar Macbook, once you add all the nice extras that the MacBook comes with (built-in camera, remote control, Apple's iLife media creation/editing software, XP Pro--more equivalent to the MacOS than XP Home--etc), you end up paying at least as much, generally more, for the Windows computer. I did direct Sony and Dell comparisons with the MacBook, and you paid about the same for a heavier Dell, and significantly more for a somewhat lighter Sony Vaio.
MacBook Pros are generally similar--I recently bought a 17" one, and configuring a similarly equipped laptop from Dell cost a bit more.
The point I'm getting at here is that while Macs are generally on the slightly priceier side, this is because they include "the works" out of the box, while most makers start with a stripped down model then charge more for the add-ons. If you don't need the extras, you save by going with another company, but you do get what you pay for.
7) Were I in your situation, I'd probably try this: Buy a not-too-expensive Apple and also install Windows on it. If you end up loving the MacOS, you're in great shape. If you end up hating the MacOS, then just boot in Windows and pretend it's not a Mac--you've basically just got a highish-end Windows computer with an Apple logo on it at worst.
Note also that while I've been talking about laptops, the iMacs are really, really great machines, and if you don't need the portability, I highly recommend them. You could also just try grabbing a Mini to test the waters--they're not that expensive, and can use an existing Monitor and USB keyboard/mouse that you probably have around. Use it as a media center/DVD player/jukebox for your TV if you don't end up liking it--they're quite good for that.
