I can't see this ending well at all. eBay is going to let sellers list the newly-updated iPad ahead of its release next Friday.
Under eBay's pre-sale policy, items can be listed that the seller doesn't possess provided they are dispatched within 20 days after receiving payment. On top of these rules, there are restrictions on the number of iPads that can be listed in both launch week and subsequently, and controls to stop new sellers creating spurious listings.
Even so, I suspect there will be some disappointed arguments in the weeks to come, as well as many people paying over the odds. The prices on the items listed are already ludicrously high in some instances (check the picture for an example, priced $600 above the US market value).
iPads are undoubtedly going to be in limited supply at launch (whether that's because of component shortages, careful management to maximise consumer excitement or sheer raw demand isn't really the point in this context). I can't see that ordering from someone on eBay makes much sense when Apple itself sells the device postage-free, but I suspect that won't stop some prospective iPad owners from getting sucked in.