headlights on, but nobody home

[dlk]

330ci.

Offered a 2000 330ci... it refused to start off the regular battery, but jump started fine (hooked up under the hood rather than via the battery terminals).

Flat battery? Doesn't seem so, since there seems to be good power to appliances & headlights. And bizzarely, on attempting to start with the regular battery, the engine would not crank but the headlights came on (even though they weren't switched on).

What's the deal?