With A used bike unless you know the owner, you really don't know how he's cared for it? Usually no warranty left on them, unless they purchased an extended plan. So with that being said the price has to warrant the lack of knowledge of "care" and no warranty and how many miles are on the bike. Good thing you can find ducs with low miles and not beat to bad.
New bike you pay more and you you don't worry about how it's been treated, maintained. Full warranty, it's new
Most people who buy only new or used usually stay in their comfort zone and repeat.