How much do you reckon I could ask for 4 brand new 18" Austin alloy wheels on Bridgestone tyres. They have covered just 150 miles so far!
I'd see what the average price is on ebay for them and knock 10% off (ebay fees) if selling through the forum. If they're priced right then they will sell.
You might want to try some of the more generic forums too.
Here you might sell yours to someone who wants a spare set, or sell one at a time to people that have wrecked one.
Over on a normal Golf forum you'll likely get some interest from someone in a non-performance Golf who wants GTI wheels. I presume this car is wholly yours to sell? If not, giving it back at the end of a PCP rather than going the p/x route might be a no-go. P/xing might see you get less money without the right wheels.