1998 Ford F 150 Automatic 4wd Recently Purchased Seller Electrical Problems

I have a Ford F-150 1998 Automatic 4WD. I recently purchased it. The seller said it has "electrical problems" we had to jump it originally. I left it running for a while, bought it and took it home (1-hour drive). I got home turned off the truck, came back about 5 minutes later and it wouldn't start. When you put the key in, lights come on, headlights work, and the door dings. When the key is turned, it clicks ONCE, and everything is dead, no lights no sound, you turn the key again and nothing. nothing at all. EVERYTHING is dead. The battery tested good. I replaced both battery connections, and there is no corrosion at all. The truck can still be jumped, but it cannot start under its own power. Any ideas on what is going on? Any help would be appreciated