If it is a bike that is NOT stolen, fits you well, is in good condition (no worn out or cheap parts) and does not need a tune up and the seller is asking a reasonable price, maybe go for it. You won't get any warranty with it and you won't know the history of it and what might have happened to it unless the owner is really truly upfront and honest.

Never buy one of those online bikes or anything you can find a department store because most of the time you can get a brand new pile of scrap metal from whatever place they bought it for not much more.