Andy im not sure if you already know how to do it, but to create a link in the forum to say a website or an ebay listing, click on the address at the top of your browser window, it will be something along the lines of:
http://www.websitenamehere.co.uk
It will turn blue, press Ctrl (bottom left) and the C key simulatanously. This has copied it.
To paste it into a message press Ctrl and the V key simulatanously. The forum will automatically make it a clickable link.