Good to hear you're all set to dive in, although I am wondering if you've taken your listing fees into account. You're talking about 500 to 1000 items, you have to be fully aware that eBay are going to hit you with a listing bill whether you sell anything or not, and it won't be pretty with that volume of listings, no matter what price you start them at.
This is ringing alarm bells because I know of a member who did pretty much the same thing just a couple of weeks ago without thinking of the listing fees, then got hit with a 300 odd dollar bill without selling a thing.
Just to get started, I would suggest numbers closer to 5 or 10 of the items you feel more comfortable about selling. You mention here that you have discovered a niche, I'm sure that niche won't contain any where near 500 to 1000 products, so I would suggest focusing more on the core niche products themselves and build up from there.
If you really want to jump into the market with that volume of items, I would suggest looking at free listing sites, bonanza for example. You won't get the same traffic flow as you will with eBay, that's true, but you won't get the sting in the tail just for listing either.
Happy to take a look at any listings, just leave a link we can follow for us.