I would approach them the same way you would the mortgage holders. They still need to release the lien or your buyers won’t be able to secure financing. If the deal is good and the lien is not much, you could pay it off yourself. Just make sure you get a release before you pay it.
Check your statutes, or ask a title company about how long they are valid. In my state, Mechanic’s Liens expire after just one year if not renewed, so it may be a non-issue.