What happens if you win a government auction and don't pay?

It is a default. A winning bid is a binding commitment, and if you do not pay or collect inside the deadlines you can lose the item, owe damages, and have your account blocked.

The penalty depends on the platform. On GSA Auctions, failing to remove the property costs liquidated damages of the greater of 20% of the price or $200, you lose the item if the default is not cured within 7 calendar days, and unpaid damages block you from bidding. On Public Surplus a default can mean a suspended account and a forfeited deposit. On GovDeals late payment can lead to suspension. GovPlanet charges a default fee and a per-day late fee, both tiered by price.

Do not bid on anything you cannot pay for and collect in time. If you win several lots in a week, map the pickups before you bid on the next one.