Ebay is great.
Make sure the seller has good feedback, say 100+ transactions with 99%+ positive feedback.
I buy and sell loads of stuff on Ebay and the only problems I have ever had were a couple of people buying something and then not paying - I still had the item so I left them bad feedback and put it up for sale again; I just lost the Ebay commission.
U.S. Ebay (ebay.com) is much bigger on the aviation front than ebay.co.uk and you can buy and sell there too. It's all the same Ebay, the prices are in US$.
But you do need a paypal account. Without paypal, Ebay is a hassle.