There are many types of liens. Some may be easily corrected, such as a contractors lien that has been satisfied but was never removed from the record. A phone call can clear those up. Other liens, such as bank, tax or HOA liens are very serious and need to be addressed before a seller can deliver marketable title to a buyer.
You would want to consult a RE attorney, but I guess you could either make your offer contingent on the lien being cleared or buy at an appropriate discount and pay the lien yourself.
Ignoring a lien against the property is not a solution.