It's perfectly fine fluff-wise as long as in-game you're not trying to say the model has Termie armour and a jump pack

which doesn't sound like the case from your post. It can be tricky to do, but if done right it will look amazing. Really, you want the wings to go more or less straight up and back rather than curved, and protruding from the top panel or the groove between the armour/shoulderpads rather than from the vents on the back.
As for resale value - if you're looking to sell it before you've even made it, I wouldn't bother. You'd need to be an extremely good modeller *and* painter to be able to make this attractive to someone who hasn't thought up the idea themselves (and therefore knows how they want it). Basically if your painting is that good then you'll know from your previous sales already that you could pull it off.
I don't mean to rail on one of our own poor members(!), but in my opinion this is how it should NOT look, the wings don't look like they're in a natural position nor do they look like they're big enough to do anything useful:
Unfortunately I couldn't find any decent examples, and only one or two more examples *at all*, it seems very few people have actually tried this.