Of course you'll always find people overpaying on eBay - that is why you can find an M9 that sold for $4999 recently. That was my point to the OP - you generally can't look at a single low (or high) price on eBay and claim that is the market. And you surely can't expect M9 prices to drop below that of an M8!
IMO, anyone paying $2000 for an M8, unless it is brand new with warranty, is over paying by at least $300-350. I arrive at this by looking at eBay to see what the lower ones are going for and then comparing to other forums where there is a larger trust factor in making a deal (like RFF). If M8s are posted on forums by established members for $1700-1800 and they don't move quickly, then they are priced too high, in general.
The M8 is getting very old in digital terms, but it has held its value very well. But right now, IMO (and I am considering getting back into an M8, due to the price drops), those in the market for an M8 should be looking to pick one up at a target price of about $1500. If you are impatient, then plan for $1700 or so. $2000-2100 would be my target for an M8.2 (not upgrade!), but I probably do not see enough advantages to pay the extra $500.
If you're selling an M8, and want the hassle of trying to wring out every last dime, then go to eBay and put a 30day auction out there for $2000 or Best Offer, you may get it. But if you want to sell it quickly and much more safely, then post it here for $1500-1700.