Anyone knows what could happen to the IRS lien after purchasing property at county tax sale?
IRS Liens are always junior to the property tax lien and are therefore wiped out by the tax sale. Except that when a property on which the IRS holds a tax lien is sold at public auction, the United States has the right of redemption for 120 days from the date of such sale or for the period allowable for redemption under local law, whichever is longer (26 U.S.C. §7425(d); §3712(g)). If they do redeem you will get your purchase amount back.
As a practical matter the IRS rarely redeems, but if there is a IRS tax lien it is generally a good idea not to make any additional investments into repairs, etc until after the redemption period is over.