buying property at sheriff sale with federal tax se...

Want to buy property at sheriff sale with a federal tax secondary lien, will I have to pay that second lien off too?

You’ll want to check the laws in your state but my guess is that the Sheriff’s sale will wipe out the IRS lien… except that the IRS will have the right to buy the house back from you for what you paid for 120 days (the redemption period).