Can't remember if the GPUs have any, been a while since I've had the heatsink off one, but I would remove any electrolytic capacitors by hand prior to reflowing them if you can.
You have to gauge your soldering skills here. There are a lot of caps on a video card, so what are the odds you'll mess up something else doing all that desoldering/soldering, and there have been studies that have shown that reflowing electrolytic caps have no influence on their long-term degradation.
Using a heat gun is a good option too, as long as you know what you are doing, but once again there's risk here - you hear stories about how folks overheated a chip and it falls off the circuit board...
When selling you need to be honest about the products condition, but I think home oven reflowing is a good option, especially with all the reliability issues lead-free solder has brought (okay, it's better for the environment), and if it can keep your video card out of the landfill.