I am not a fan of buying used stuff but it can be a good way to save money. The problem is that it often ends up costing nearly as much as going new anyway. Buy local, and buy from friends or at least someone who is a friend of a friend so you get an honest description of how it was used.
For a used FS frame I would factor in the cost of immediately replacing all bearings and pivot hardware so you know you are starting with a clean slate.
For a complete bike, factor in the possibility of needing all new drivetrain components (at least a chain and cassette, if not rings too) as well as brake pads, maybe a new handlebar to be safe, maybe tires, cables, different weight springs front and rear (unless it's air), etc.