YES! I'd take it at an even lower percentage. (Some might buy the speed upgrades only while needed, which I can't picture the devs allowing at such a good return rate....) I'd particularly like the option to recycle my character upgrades (as opposed to skill); I'm frustrated with the lower energy level and needing to eat, but loath to go up as high as I was before, lest I regret it for other reasons. I'd definitely like to go higher and then be able to drop again if I feel it was a bad idea.
Another vote for this form of recycling. There might have to be a time delay before you could resell, to prevent people from buying upgrades just to get a new card and then reselling immediately.
Actually, I'm not in favor of them paying us money back. I see it more like Unlearning. I'd even pay them a little bit to remove an upgrade. I'd just like to be able to remove a "benefit" that wasn't actually benefiting my playing style.
For the energy tank issue, I agree that something much more like Unlearning would make more sense. Certain upgrade cards would increase your energy tank, others would decrease it, all of them would cost iMG, as you have to imagine the change to yourself and your world (I'm picturing Alice nibbling mushrooms to grow and shrink, now). It might also mean that with the new changes, the Devs should take a look at the energy / mood ratios and tweak the math a bunch for better balance.
Also remember the cards have been out for about 3 days, and the staff has already killed some bugs and made some minor changes, and clarifications of wording and working and such. They've promised an encyclopedia of cards, which will hopefully prevent some of the remorse based on simple confusion, though most of the basic descriptions are fairly clear (at least for native speakers of U.S.). If sell-backs are ever allowed, 85% is way too high, especially if you've been using the card for a while.
I agree, sell backs are probably not the way to go, and I don't mind paying more iMG for my downgrades, I would just like the option to do so.
Also, I totally understand that the whole upgrade system is new, but if we don't discuss what our opinions of the Beta are, then we're not doing our job as testers.
+1 for an Upgrade Card to remove one upgrade; that's a really good solution. I would think that the removed upgrade would then be back in your "deck", since that depends on skills, so you can change back and forth as often as you get the right hand, and want to spend the iMG on these things.