Question Where to sell new GPU?

As far as how much you could reasonably expect to sell it for, depends on your specific location within Europe, along with availability of the product locally/supply & demand. Prices vary dramatically (it's 680GBP in the UK, 740EUR in Germany, 780EUR in France, almost 900EUR in Ireland).

eBay etc gives you a worldwide audience, but shipping/fees can be tough to swallow - and import fees for the seller can reduce what they're prepared to pay for the card initially.
I would start with a local sale - Gumtree, Craigslist etc (where avaialble) or Facebook marketplace.

If you're in a country with easy access to components (like the UK, or Germany), new from a retailer, it'll eat into what you can charge.
If you're in a fairly remote area, you could probably get near retail for it....