I have a bike I bought off eBay and another built almost entirely out of parts bought online. I bought my wife’s bike on a significantly reduced last-year’s-model price off Amazon.
Don’t go into it expecting to get your dream bike; you will have to make some compromises. Buying everything new will reduce headaches. Double check the conditions of whatever buyer-protection system you’re using.
Study the pictures carefully, and ask the seller for more details about anything you’re unsure of. Ask again if their answers are in any way vague.