Firstly, if you have to re-sell themes then you need to be sure that the licence permits it and that you are being open with the client, no buying a theme for $20, claiming it is your own and charging the client $1000.
You should learn how themes work and look towards developing your own to become a web designer, simply re-selling themes makes you a middle man, not a web designer.
An honest approach to this is by selling your services as technical assistance, you have your client pick a theme or you choose one based on their requirements and do the installation and set-up for them, alternatively learn how WordPress themes work and offer your services as a web designer to re-brand and modify the purchased template.
As for template sites, I like Theme Forest however there are loads of them out there however I cannot stress enough, make sure you are not breaking licence or market place usage terms or you could find yourself in trouble.