Normally people post them under long guns in the classifieds. Be sure to state the obvious stuff- Location, disclaimer, sale price, NFA item, MG license required (or not, if it's not in MA or not an MG) what kind of form it's on, etc. I would also try to express in the thread title that it's an MG, AOW, SBS, etc. eg: "FS: Pre-86 Transferable RR M16 (NFA registered MG) in MA w/accys "