The selling of ISK for cash is frowned upon by CCP, and can result in an account ban. Buying ISK for cash is against the EULA.
However, buying Time Codes is a legitimate way to pay for your account and these can be difficult to track down. Some players sell Time Codes on the forums for millions of ISK, or you could use one of the listed companies advertising here.
If you would like to advertise your Eve Time Code sales here, please email us here.