Unless you 'really' need the money from selling your DK2 now to fund the purchase a CV1 pre-order - I wouldn't sell the DK2.
2-3 months without VR would be bad enough, but what if....... CV1 ends up unsupported for ED after all (so you end up selling it/cancelling the order), or the HTC Vive is delayed again, or the Vive price is a lot more than you thought, or you find that the hardware required by your pc for CV1/vive to run smoothly is more than you first thought (when I had a go on the vive at EGX, the pc was running a titan GPU setup - they ain't cheap). There are still a lot of "if's".
At the end of the day - the DK2 works now without 'massive' hardware requirements and will likely do so for the forseeable future.
As for DK2 e*ay prices - I paid £300 back in Jan 2015 for a 2nd hand one. I could still get that price now as the demand is still there. The price of the CV1 is more than many people thought it would be (as can be seen in the many posts on this forum) so newbies may opt for a DK2 instead - driving up demand and keeping prices high. The vive looks to be a more technological set-up (compared to CV1) from what I've read online so I can't see how Vive will be cheaper than the CV1 when its released (I predict it will be a fair bit higher than the CV1).
VR is a new concept for the mass consumer market and is a niche market at present.
I doubt that VR prices will suddenly drop anytime soon - why should they (especially if you need a reasonably powerful pc + hardware to run it). Many newbie VR people may be put off the CV1/vive because of the price/hardware requirements. This in itself will keep DK2 prices high for a long while yet as newbies jump on the bandwagon so it could cost you as much (or more) if you decided to buy back a DK2 in the future - compared to if you were to sell it now.
Just my own thoughts on the subject - others may disagree.
CMDR Blastard