If you are planning on selling or buying a pre-order, I'd recommend against it. From the seller's standpoint, it'd be hard to convince somebody that you aren't a scammer. What's more, what if your Gamestop/EB doesn't get as many PS3s are they are supposed to? You'll be left without a system for your buyer.
From the buyer's standpoint, it'd be foolish to buy something that isn't yet in the hands of the seller. I'd recommend that if you want to buy a PS3 from eBay or Craigslist that you wait until the seller has the PS3, has taken pictures of it, and assures you that he/she is selling the unopened system, not just the box. I say this because during last year's Xbox 360 eBay sell off many sellers took pictures of the actual box the system came in and then deceptively worded their listing so that unwary buyers purchased just the box itself, not the system inside.
If you really want a PS3 and don't want to stand in line, then yes, eBay or Craigslist is a good way to get one (if you have the money). However, be wary and make sure you read the listing carefully, look at the buyer's rating, make sure that the seller will ship with tracking and insurance, and use PayPal.
If you're using Craigslist, you'll be local, so it'd be worth the drive to go see the system that you're buying.
Good luck you non-preordered, unwashed masses!