If my 18 year old son is getting charged with selling drugs for second time, what will happen and am I liable since he doesn't live at home?

When you say he is being charged with selling drugs for a second time, does this also mean that he has been previously convicted of drug possession in the past? If so, then he was probably placed on some form of a probation with the court. In this current matter, the court can take into account any prior drug convictions your son may have, which means that he may be facing heightened scrutiny and sentencing before the judge for a second offense. My advice is to contact an experienced criminal defense attorney in your area to represent your son and help him avoid any jail time and to even help protect his criminal record from this new offense.