I'm watching one on ebay, however it's been used in a beauty salon, may seem a stupid question but would this be ok to buy for cake decoration or would this be deemed as unsafe? I have never used one and I'm not sure whether it could be cleaned from contamination?

Is this an airbrush compressor? If it's just the compressor you will need to buy an airbrush with connecting tube so it's safe in my opinion. Just give it a clean with anti-bac. If it's the whole set up I'd still buy a new airbrush and connecting tube. 

I don't see why it wouldn't be if you got new hoses. To my understanding it compresses air so what ever they used, paint, etc would contaminate the hose and the airbrush gun I think. I don't know if there is a filter inside it that you could change.

I bought mine used and bought a new air regulator, hose and airbrush. I never thought about contamination from what the previous owner used it for !!!!

Maybe RedneckRuffle will pipe in with some answers.