I could be wrong but I suspect that the reduction in the number of Wii systems available to buy may just be because scalpers are finding the work necessary to find a Wii system is greater than the return available on eBay. If you can only get one system per store, and you have to line-up at the store for 1 hour (at least) before it opens to buy that system (and with all the other work you need to do to sell it) how much do you have to sell the system for to make it worth your while?
With a lower number of systems available the price per unit goes up because there is greater competition for every unit.