Sure you can control fan speed. Fan load is inductive, thats why you need connect snubber circuit (RC filter) to triac output. Without snubber, you can damage triac.
On that eBay page there are loads of links to YouTube videos and example sketches for just this issue. Have you tried any of them? And with what success?
It does also state in bold letters on that eBay page (I'm paraphrasing here): Not knowing what you're doing when it comes to high voltages will end with you in the morgue. Seriously.
It's too easy to touch some live part of a high voltage circuit whilst 'developing' so for your sake, take the utmost safety precautions.
Post back when you have tested out some of the example sketches!