1: I dont see how a server can be 'legit' if you dont have the interest/skills to continuously update to add new content, features and fix any issues that arise (there's no such thing as a 'completed server', there will always be tweaks to make)
2: There is a sticky thread for all buying/selling/hiring/volunteering/etc -
3: If someone truly has a 'ready to host' source, they will make far more running a server than they would selling it to you. Selling a source is never worthwhile unless the source is pretty ****** to start with. Essentially any offer to sell a source is always going to be a scam in the sense it's only ever worth selling if you're charging far more than its worth.