Why does the winner pay the second-highest bid?

To keep bidding honest. This is called a Vickrey auction, named after William Vickrey, a Nobel-winning economist.

You bid as much as the item is worth to you, no more, no less. You don't know — and don't need to know — what others bid, only what you'd be willing to give. This is also called honest bidding, and it's impossible to game the system either way.

If you lose the bid, someone needed it more — and you save your seeds for next week's offers. If two members bid the same, the first to bid wins and pays what they bid. If nobody else bid, or only bid very low compared to you, you pay a very modest amount — down to 1 seed.