By agreeing with PunkBuster's EULA, the seller allowed the actual computer to be hardware-banned if he was cheating. He got busted and the computer became useless (to gamers at least). So he decided to get rid of it by selling on eBay or somewhere. Therefore, he knowingly sold you a useless computer.
It is sort like knowingly selling a bad or broken car (a lemon) to a buyer. Its the seller at fault, not EvenBalance.
Plus, EvenBalance banned the computer, not you. They are not stopping you from using other computers to play on PB-enabled server. EvenBalance just don't like the computer itself (which is the one that violated the EULA in the first place).
However, I would be very interested to see how this goes in court.