Buying ISK for real money is not allowed!
Buying ISK for real money is a EULA violation and a punishable offence. The ISK being sold usually comes from hacked accounts, exploits and macroing. The ISK sellers have many tricks up their sleeves, such as rigging their websites to place key-loggers on their customers' computers in order to later hack into their accounts and remove the ISK they just sold, and any other items of value belonging to the players, their corporation or even alliance. This is NOT fiction or exaggeration - this is an actual and very recent case.
Do not support this sort of thing by buying ISK!