Many of the Dell laptops sold are brand new product with full USA warranty. I've asked the same questions in the EBAY forums. I recently purchased a Dell 600 on Ebay for like $900 and it retailed close to $1,300.
If you read the posts carefully, you may start to notice a line in the purchase order (most post them for computer specs) which says "Dell Soft Contract". I am sure there is some "wholesale" program maybe based on quantity that allows for a decent profit on Ebay.
With Toshiba and similar manufacturers, they may "drop ship". Where the person selling on Ebay doesn't actually "buy" the product for resale. They sell it on Ebay and put the order thru the manufacturer. The manufacturer ships to the buyer directly and bills the Ebay seller (who's already collected payment and shipping from buyer).
There are people on Ebay actually selling the "lists" of these wholesalers. It is probably very profitable if you know your product. I would think 95% of those lists are bogus. BUT someone, somewhere is making it happen.
I am always careful ensure that the products I am interested in are genuine USA products (I live in the US). If they aren't (considered grey market), the company may not warranty it, because it was manufactured for resell outside the US only.
I don't have solid answers, but this has been my experience with Ebay. Just be VERY careful when pruchasing big ticket items on Ebay. In the forums there is some good info on scams and whatnot. Information is very valuable.