My concern is at this time it looks like the resale value of Macs may have gone down unlike 5+ years ago. I am seeing eBay sale listings of the new Mac Pro being offered at low prices like HERE. And this may also affect the resale values of older Mac Pros as well. In the sale ad, the New Mac Pro, a 6 core 3.5 with 64g ram is being offered at $4500 for Buy It Now and at $3400 to bid plus it has AppleCare up to 2017. I don't think these sellers are selling for profits. I think the new Mac Pro's sales are good. You'll just have to take note of the resale value should you decide to sell it after some years.
A starting bid price doesn't really mean anything, and a $4500 buy-it-now price on a $4000 computer with a ~$700 aftermarket memory upgrade and AppleCare is only about 10% below full retail price.
There was an old sale ad months ago, a 12 core New Mac Pro that was sold for only $7k also with AppleCare. These are good deals considering this is the current model. If the sellers outnumber the buyers, then that may bring down the resale values not only with Mac Pros but other Macs as well.
There's a sort of "It loses 20% of its value when you drive it off the lot" phenomenon with new gear. Why is someone going to pay some random eBay seller the same price they'd pay Apple, for a system that they can't build to order, that has some hours on it, that there's a chance might not be accurately represented in the listing, and so on? Of course you're going to want to save 10 or 20% over Apple's prices; you might as well just buy from Apple otherwise. I think you may be extrapolating too much from this initial hit.