Neutral start switch or shift rod bushings. Do you have a column shift or floor shift? In either case, lift or push the level into park and hold it there while turning the key, see if that gets the starter spinning. Try it with the selector in neutral too.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.