Is it safe to use an 85watt adapter from a 2006, 17" macbook pro on a late 2011 15" macbook pro?
I recommend you buy one directly from Apple. There are many counterfeit power adapters on the market that could damage your MacBook Pro or potentially cause a fire.
Any idea how to check that an adapter from ebay is a true / legitimate / authentic apple adapter?
Nearly all of them are, even if the seller insists it is "genuine OEM". Legitimate adapters from eBay are likely to be used, not new. There is nothing wrong with used, as long as you know what you're getting.
Identifying a counterfeit is possible only through close examination.