Will not crank over

Purchased this vehicle from private party. They said they had only used it twice, when the starter failed. The positive connection had so much corrosion on it I thought this was the problem. I replaced the end and tried to start it. When I turned the key to crank it, nothing. All my dash lights worked, along with the gauges etc. Nothing from the starter. I jumped across the main posts of the relay on the firewall and the starter cranked over nicely. Nothing with the key, however. Where do I go from here?