Would people actually buy it? How much should I sell it for?
You could probably sell it for 900. I'd expect at least a 100 dollar loss but I'm almost certain it would sell for that. Maybe 950, but I don't know for sure. Gift cards usually take a bigger loss the larger the amount it is.
However, it might be better to just buy a full-retail price and sell it as brand new, as your original plan stated. An iPhone 6 Plus, if you can find it, could sell for well above 1,000 if you're able to find one in the wild in the month. Aside from that, maybe a MacBook Air are probably your only options to buy-and-resell at a similar value.