Personally, I find the best deals by selling locally (craigslist, letgo). But this is often dangerous for the inexperienced. Ebay is the route I would take. You can see what the current ones are going for, and put yours at a lower price if you want it to go faster. You can go to re-sellers like gazelle, but they honestly dont give you as much money for it because they will simply buy it low to sell it higher and make a profit. Youll make the most money selling to a direct end user who wants a 12" ipad pro, and not a re-seller who is going to pay you less than its worth.
Though if I may offer my unsolicited opinion on the matter, I probably wouldnt make the move to upgrade from the 1st gen to 2nd gen for the sole sake of 'future proofing', mainly because there is no telling what the future holds and how long the devices will actually last. I completely understand the want for the newer features; but with the resale values of the 12" pros that Ive seen, and the prices of the new ones, I just cant see that price difference as a good value for the differences. But that is just me.