for about $600 you can get a decent bike from Specialized. it will have crap components but it will have a decent seat and it will actually be a real bike.

i think used bikes are not worth it because people will buy a bike for $3,000 then 10 years later you can buy the same quality bike new for $600 but they still remember paying $3,000 for it so they will try to sell it for $800 even though it doesn't even work any more.